I had this problem occur in a G36k, because the trigger contacts were jammed together. This may be his problem, but I have never worked on a SCAR, nor have I had this problem with any V2 gearbox so I'm not 100% sure this is the problem.
If it is, it will require taking the whole gearbox apart, which, from what I've heard about the SCAR's take down, isn't the most pleasant of tasks.
Unless it suddenly starts working again, I suggest he take it to someone who is an experienced airsmith who can help him out, specifically someone who knows how to take a SCAR apart as they are unique in their take down, but the gearbox is very similar to a CA M15 CQB.