I had something like this happen where I used to work.
Do not cold approach this girl. You've seen her, and guess what...She's seen you.
Next, time you see her say hi and walk off. Do this every time you see her. 3 is good and on the 4th time you open her up indirectly by asking her how long has she been going there or whatever and how she likes the college. Then bounce.
Finally, the next time you see her you can invite her out. If you have any friends you should get together with them and do something then invite her along. If you don't have any friends then your friends canceled on you when the meetup happens. Matter of fact...The friends canceled on you regardless is the best thing so you'll get her alone. I would say invite actual friends if she insists on bringing her own.
The boyfriend. I can't remember where I learned this, but what I like to do is to talk about the boyfriend. I'll ask all excited and stuff (I really do get excited, it gives us a nice thread to talk about for a while) "When did you meet?

" When she answers she'll go into her memory if there really is a boyfriend. Since she is already in state with you all of the emotions she felt then when he gamed her will be added to what you've already built. You can talk about him as much as you want, but then you have to transition into asking for her number again.
I usually just tell her we can invite the boyfriend along to whatever plans we make. (later through text I set up a 1o1 thing) It doesn't matter. The point is to get her phone number. You can go direct on her later, but right now you just need to ease your foot in the door.
Hope this helps.