<human>: There's a woman in my circle of friends who is very obviously attracted to me.  It's totally mutual.

Thing is, she has a boyfriend.

I haven't known them for very long but he's friends with most of my friends.  They all went to college together.

On halloween, as she and I were talking -- and pretty much tuning *everybody* else out -- I became self-conscious and thought "what the hell am I doing?"

The whole room had disappeared except for her and suddenly my thoughts turned to her boyfriend.  He's a pretty cool guy and has known most of my friends since forever.  

And I didn't want to embarrass him.

But he didn't seem bothered at all.

He and I talked for a while when she went to the bathroom (it was about a 20 minute wait to get into the bathroom) and after she came back and started dancing -- he excused himself to go to the bathroom as well.

But before he left, he pointed out where she was on the dance floor.

Anyway, all night (we didn't part ways till dawn) she was super flirty with me and he seemed perfectly ok with it.

And now I'm thinking maybe one of two things is going on:

 - their relationship is open and I just need to stop stressing out about it
 - she's getting ready to jump ship and he no longer cares

So I don't know how to proceed.  Can I just ask her if she's in an open relationship? (that's my current plan)

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: mutual attraction with friend who has a boyfriend.  A fling may be acceptable but I don't know how to ask.