I'm totally new to the forum here, and I am no expert, because I learn new things every day, but I am a firearms instructor and run across these things with new shooters a lot.
1) I know several "lefties" that have no issue working slide stops with their "strong" hand and giving them a lefty made gun only messes them up, because they've practiced with their weapon. I would have your friend just practice manipulation with the Glock 34 slide stop using their index finger to push up on it while it is indexed off the frame (finger off the trigger) to lock the slide to the rear. When doing speed reloads (slide lock and ran dry) there's a couple of options. They could either load the fresh mag and then grab the slide on the rear portion and rack it back and release to charge the weapon, or if they want to use the slide stop as a release, they could practice resting their shooting index finger over the slide stop and after inserting a fresh mag use the index finger to pull down on the slide stop to release the slide. The Glock 34 should have come standard with the "extended" slide stop and this should help with those manipulations. I have learned a lot more from lefty shooters than I have taught them about this, because I am right handed myself. These are things I've been told by lefties that work best for them so that it can be passed on to other new shooters. It all comes down to pick your gun and practice practice practice. A lefty will find a way.
2) Marksmanship comes with time and practice also. Everyone's different when it comes to shooting. I know that the majority of the time a practiced shooter can take a stock Glock and hit dead center. Others may need the sights bumped ever so slightly. This is by no means an all inclusive list of what could be causing the shooter to hit high right, here's a few of suggestions with the high right shooting to check on that are usually the first things that I look for when dealing with it. Make sure there is a good grip. High as possible and no leaks in the grip. I've found over the last few years that thumbs tend to get in the way on your grip. They do for me all the time. Try to keep the thumbs out of the equation until the two hand grip has been established and just let the thumbs rest there. High right with a lefty can be cause by a lot of different combinations, but here are some of the most common I have seen. A) Heeling - it's a type of anticipation but instead of the muzzle just before the shot breaks, the shooter actually is pushing with the heel of the "strong" hand just before the shot breaks kind of in a subconscious effort to help the bullet out of the barrel and to mitigate the felt recoil if you will. Milking - as they trigger is squeezed back the shooter starts to increase the grip with all fingers of the shooting hand (so squeezing with their entire hand instead of just using their trigger finger) and this can cause the muzzle to go up and to the right as the shot is being broke. C) Poor sight alignment coupled with poor sight picture and trigger control. For this one a lot happens and it can be frustrating when you're trying to help the shooter correct the issue, because you yourself are trying to figure out the issue in your own head. First, the shooter may be trying to line sight dots up instead of having good sight alignment where the top of the front sight post is level with the top of the rear sight and centered or they may not be focused on the front sight post to tell that the front sight post is too high and they may be focusing on the target more than their sights. For marksmanship the front sight post has to be perfectly clear. Next, a lefty that doesn't have enough trigger finger on the trigger will actually push the muzzle of the weapon to the right as they are squeezing the trigger back. And they could also be jerking the trigger at the last possible moment and that can be hard to see. They'll take up slack, squeeze little by little, but right before they shot breaks they jerk it because they're tired of waiting for it to go off. D) The shooter could have poor sight alignment and be pulling the weapon to the right with their support hand. So, they could be aiming just a little high and instead of applying pressure to the frame of the weapon with their support hand they are actually pulling the weapon to the right with it. Try some one handed drills at 7 and 10 yards to see where they hit. Then, add the support hand and see where they hit at 7 and 10 yards. These are just a few combinations of things that could be happening.
3) Grouping. If they are a new shooter I'd take a 5" group at 15 yards any day, because usually it can only get better. Grouping is one of those things that come back down to your sight alignment and sight picture with trigger control and follow through all in one. The biggest thing I find that hurts people is they don't focus on the front sight post so that it is crystal clear and the rear sight and target are fuzzy. The second thing I see that cause large groups is that the shooter will fire a round, look to see where it hit and then chase their holes all over the paper instead of keeping the same sight picture that they had on the previous shot. You also have to keep in mind that those things that could be causing the up and to the right are amplified by distance. So at 3 or 7 yards they look like they're drilling the center, but the further back they get the more pronounced the issue becomes. A 1/4" of movement at 3 yards Vs. 1/4" of movement at 15 yards make a big difference.
If you're teaching your friend fundamentals it all starts with grip, sight alignment, sight picture and trigger control. Dry firing is worth its weight when it comes to instilling muscle memory to the fundamentals, but I have found over the last 6 years instructing, that inevitably there has to be the boom and when the boom comes your subconscious will take over and produce hiccups. There is no replacement for live fire because that is when they will learn to track their sights through the recoil and learn how to mitigate or control recoil. Try doing live fire marksmanship drills with dummy rounds mixed into the magazine. Load a 15 round magazine with 6-7 dummy rounds. Don't let the shooter load it though. You load it and give it to them. Make sure the first and last round in the magazine is a live round. Sometimes, when a shooter sees the gun move during this type of drill they know that they did something subconsciously that needs to be corrected and they learn more this way. Not all the time though. There are other drills, but this is one of the most effective that I have found to use when teaching a new shooter.
Sorry for rambling on, but I hope that some of this can help you out if it makes sense. Let me know what comes of it.