Video evidence sounds like the best option here, but if you were online during the time of the match, see if you can find the specific match on Battlelog. If we're lucky, there's a chance the stats were saved and could be a way to prove who was the cheater.
Oi, legit question now. When someone does the invisibility glitch with the intention of abusing it, is it how they look in first person? Like, no hands models on the weapon?
Truth to be told, I'm not sure if I think Red Alert 3 was shit because it killed my favorite kind of offline gameplay or if because EA is shit themselves...