The Rulebook says that if a competitor "prematurely starts, there will be a 3 second PE assessed". However, I see it as if you have the time to recognize an adjacent squad begin shooting, and to look back at the SO in between the SO commands, there is way too much delay, and it could feasibly be construed as an issue with the timer as recognized by the shooter. I have seen this happen with delay settings on timers as opposed to instant starts, or buttons sticking. As a SO, if it was a local club match and I was the SO, I would have stopped, disregarded and started the process over again. If you draw on the other hand, the SO has the sole discretion weather to STOP or continue the course of fire.
If a sanctioned match, I have always seen SO's go out of their way to aid the shooter with commands, and I cannot accurately comment on how the situation would be handled as it would be an item to be reviewed with the match director.