To answer the question. I believe you lose EVERYTHING if you hit fresh reset, at least that's how it's described in the multiple warning prompts you are giving prior to doing it.
I do think it's a bit sketchy, but anything that comes from the actual reserves (whether the battle tier or ones you earned via playing) I think are lost in a blackhole when you fresh start a character. However I also think if you spent MONEY on reserves and unlocked anything they too get removed which I think is utter theft (although I'm not positive on that)
Would only seem right that a fresh start removes everything you unlocked via normal playing conditions, but knowing this game they don't make a distinction. -- But on a side, how did he accidentally fresh start his character? The option is located in the opposite corner of the screen, is smaller a bit more difficult to notice and gives you 3 prompts to make sure you understand what you are about to do.
If he's never played COD before then fine. If he's prestiged before then there's no real excuse.
EoTR- (PS4 Message me here if you want to party up)