Another vote for cheating. If it was a group assignment, then they wouldn’t have been each taking individual tests. I don’t see what difference it makes that it was on a computer instead of on paper. Who cares? “Open book” != “consult your neighbors.” I can see that they might have been honestly confused about it, but if they were not told working as a group was okay, then I think they were wrong to assume it was okay. It’s not generally okay to ask around when taking a test; why would it be in this case? Because it was on a computer? Because it was in a lab instead of a class? Because it was unsupervised? Again, I don’t see those things as justifyiing the belief that the behavior magically becomes okay, when otherwise it wouldn’t be.
But I don’t see why you’re asking since he’s apparently not going to do anything about it anyway. Even if he concludes he did the wrong thing, he’s not going to admit to it – despite the fact that these six may well have taken advantage of other students who didn’t consult each other on the test.
IMO, if he thinks it was cheating, even in retrospect, he should be a man about it and admit it to his professor. Easy for me to say, I know; especially when he may well be getting his friends in serious trouble as well. So I can certainly see that he might believe the wisest course is to be silent. Bht then – again – I don’t know why you’d be asking whether it was or wasn’t cheating. Seems to me you (and he) would be happier not examining too closely his initial conclusion that it wasn’t.