<human>: A close friend has been cheating on his gf of 15 or 16 yrs (let's call her "Beth" to distinguish her from the mistress, "Missi") for the past 2 yrs with a 29 y/o (f) he met at a conference.

 Everyone has known all along but Beth just found out. (The affair was obvious, but she ignored the signs.) Friends disapproved of the cheating but pitied him b/c his ltr was sexless (she isn't attracted to him) and for several yrs he seemed unsure he wanted to break up. It seemed like a good guy who didn't know how to break up (it is his first relationship) found somebody good for him. He was our friend and we didn't want him to cheat. But he refused to decide, saying he loved both. 2 yrs in, he has a second household w/Missi and spends most of his time there.

Now Beth knows and they've been fighting for days, but neither seem to be ending it and he won't drop his side piece. I think if they stay together he'll hide the affair (or start a new one if his mistress leaves), his gf will spy on him for the rest of his life and neither will be happy.

I don't know what advice to give. I'm afraid of him accusing me of hating Beth, but I just want them to be happy. Also, I'm afraid she may harm him or his mistress if she catches them together.

Provide a brief overview of the text.
<bot>: Friend cheating on gf. Afraid that if they stay together they'll torture each other forever. Want to help but don't know how.