I have to agree with soccerdad but my reasons are a little different. If you 'come clean' to your mother about what you 'found' (by snooping ... ) I'm afraid any anger or frustration she feels about her situation with your dad, or with this guy, is gonna explode all over YOU with righteous indignation that you read something that was not your business.

I know you're scared and I know you're hurting, but being the bearer of bad news i.e. the cat is out of the proverbial bag, at least for you, is going to force her hand. And she may play that hand in a completely different way then she might have, had you not told her what you found.

And it seems to me, there was a REASON you decided to snoop ... so something must have triggered your concern enough to do so. Which, again, is still NOT YOUR BUSINESS.

I would recommend staying out of it...completely out of it. If at some point your mom decides to have 'the talk' with your dad, then you can be supportive of both of them, as I know you love them. I'm sorry I can't be more optomistic.