Does my friend have ptsd if he saw parents having sex?

Probably not: While that was probably an awkward situation, it is not likely to have been a severe, traumatic event causing a sense of intense fear/helplessness. Now if this was a part of sexual abuse that would be a different story.

Doubtful: As traumatic as it may be for a young child to witness his parent's having sex; i don't believe that this would be so harmful as to lead to a post traumatic stress disorder. However, if a child witnesses saddistic sexual practices that may be a whole different matter. I don't know your friend so i can't tell you if he has ptsd. I doubt this would cause it.