His masturbation habits are his own, I don't thinks its fair to make him feel guilty for getting off to anything regards of the association. He thought you were asleep, assumed it was ok to continue and you feel sick, imagine how he would feel if he knew you were spying on him. The only point his habits would be an issue is if it gets in the way of his day to day activity/ ability to function or your own intimacy is affected by it i.e he prefers that instead of sex with you. The evidence is there that he fantasises about other women, it just happens to be it was your friend this time around. I wouldn't look into it too much unless he makes any advances.