Actually, the situation isn't all that weird. Many are in similar ones, just like me, who will be able to relate. Imagine being in a steady, long-term relationship and suddenly your best friend tells you he has feelings for you. What do you do? Sure, the situation is not exactly the same, but it is never exactly the same. There will be people who can relate. Especially when the relationship is going through a rough patch, we are vulnerable to temptation.
If I was the friend, a really good friend, I would want her to stay with her boyfriend. Though of course I will be there to comfort her if it does go wrong, and I would love having intimacy with her, I wouldn't want her to break her relationship just for me. If I was the boyfriend, I would want her to talk to me about what is going on and I would figure things out together. If I was the girl... That's who I am in my situation. What would the girl do? What should she do? I would say she is in a relationship for a reason, and she shouldn't forget the good things she already has. This girl focuses a lot on negative things, so it is easy for her to lose sight of things that are going well. But... There is temptation, and she is very weak to it. Both having the secret, both knowing something no one else knows, bonds them together.