Sexual assault can be a very traumatic experience for people of any gender, but there is definitely an added stigma for male survivors. In most Western societies, men are stereotyped as the "protectors." In a situation where a man is assaulted, it challenges his ability to protect himself even though any form of sexual assault is never the fault of the victim. In your friend's case, the fact that he was assaulted by another man may also cause others to question his masculinity or his sexuality. This is why male survivors are even less likely to be open to others about their assault than female survivors.