MNuTz said:stupid questions!
Because perhaps he rightly thinks that if someone is cheating or has cheated the other person has the right to know and he has also said he wasnt snooping but came across it by accident.
It definitely has something to do with him, its his dad and mum, his family life may have been put in jeapordy because of his dad.
Really?
Look at the bigger picture! Let say the dad is cheating and the OP end up informing the mum, what then? They break up and he feels dislike for his dad? ....That's the most likely scenario imo, Is that what you want? I dont think telling the mum is a good idea and im sticking to that.
I dont know the way things are in that family but like i said before, if he wants to tell, i think it might be better confronting the dad first and NOT informing the mum.