What to do now?

Eeesh, so sorry to hear that!

Sounds like sooner or later, you're dads probably going to find out, perhaps even the hard way.
Perhaps you should speak to your mum about it and ask her why she's doing this, and if she plans on telling your dad at all..

I would'nt advise that you jump to any conclusions and tell him. Get her story first.

I understand it must really hurt your feelings to have found this out, I hope things look up soon.

Ashleigh x

What good would result from saying anything?

I haven't read the emails, of course, but it could be that she is playing out fantasies in the emails; bad enough, but not cheating in the physical sense.

Her conscience may keep her awake at night, in which case she may stop this behavior on her own. Or your dad may find out (assuming he doesn't already know), and they may be able to work it out on their own.

Also, there may be two sides to it, and getting in the middle could make a bad situation worse.

My wife cheated on me over several months. She ended it on her own and told me. I surprised myself with my reaction. I set us up for couples counselling, and committed to work through our problems.

We were divorced three years later. But it wasn't the cheating - it was the whole marriage that had been broken from the start, even before we met, And I'm better for having gone through it. I have a good relationship with my kids, and they have a good relationship with their mother. I learned a great deal about myself and about loving relationships.

These things have to work out in their own time. I'm sorry you had to stumble into it/ It can only cause you pain that has no real relief. But I don't think you should bring it up with either of them, for everyone's sake.