Let them use their dominant eye. I am also Left eye dominant, but right handed. I shoot pistol righty by turning my head every so slightly to the right. This lines up the sights correctly and they will do it instinctively. I have always shot pistols righty.
When I started with shotguns, we immediately identified the cross domination. Switching to shooting lefty worked better than shading my left eye and forcing my right eye to take over. All things being equal, I'd rather shoot long guns righty, as more guns are biased for rightys, but I played around with both rifles and shotguns for a while before settling in lefty. It just worked better for me, especially with movement, like for trap and skeet. Rifle target shooting was OK righty, if I covered my left eye, but I made the decision to shoot all long guns lefty, as I could do it with both eyes open. It's all very natural and works well. Training my left hand trigger finger was simple. I spent a lot of time treating a click style pen like a trigger; Click-click, click-click. . . . . .
jack