<human>: Can you condense the text into a few key points?

I have known both for two years. 

I had always assumed their relationship was perfect and that her bringing up breaking up was just a passing thought, sort of a joke. 

I am unsure how to explain. I usually try to stay neutral when she brings this up, and defend my (guy) friend to explain what I could perceive as his side.

We have been hanging out recently more often and this topic gets broached. Last night we were hanging out and she explained her reasoning on the subject more, and now I can see why she would consider it.

I am unsure how to handle this situation, they are both friends, I gave my advice, for her to talk directly about her issues with him, but I don't know how to handle this situation?
<bot>: Friends with guy and girl, girl seriously considers breaking up with guy, I don't know what to say. Help?