I like this girl but she has a boyfriend

She's already expressed that her relationship with her boyfriend is going south, you've also discussed the topic of cheating and what would happen. If you don't make your move now, you're gonna end up waiting 'til June being filed under "friend" and then you've lost your chance. Either act now or just be happy with friends.

Women are fickle. Yeah, they'll say they're contented with what they have now, but if they're convinced they've found something better they'll leave that relationship.

Also, women are attracted to confidence, not assholishness, but confidence. This sort of meek, "Well, I don't want to ruin something you might have going on now" shit isn't going to work. You have to convince her that it's better than what she's got now.

I think the whole problem with your approach is that you're hanging out with her as friends to begin with instead of just approaching her directly. It's not difficult or intrusive to just ask. The worst she can say is "No."

Jam it back in, in the dark.