What would happen iff?

in december my friend got into a street bike accident and hit his head very badly. he has a mild case of retrograde amnesia and a mild case of post traumatic stress disorder (PTSD). so, tonight he is going riding for the first time on his street bike since the accident. with PTSD he has sudden flashbacks of the night he got into the crash and he can not snap out of them, he had one while he was surfing and he fell off of the surfboard because it affected him to that level. i was wondering what the chances of him falling off the motorcycle due to a flash back were? does he have a good chance of having a flashback while he's riding? what happens if he hits his head again will he have a more severe case of amnesia? any information you can give is helpful, thanks!