Er, well DON'T get all defensive if she tells you she has a bf, and go "Woah you dont need to tell me all your problems" or whatnot. Even said in a jokey way, it's kinda odd and rude, and it will probably cause some sort of arkward silence or she will be annoyed. I know I would be if I mentioned I had a boyfriend and someone just randomly said that.
She may have a boyfriend or she may not. Friends are weird, and they get jealous really easily. Some of my friends have lied to guys I've fancied or lied to me about guys I fancy, I wouldn't believe it 100% exactly. Did she not mention a boyfriend at all to you? That's odd... I mean I don't expect her to go "OMG and then (insert boyfriend's name) said this!!" but some sort of acknowledgement...
Basically just keep going and developing a friendship, as you were doing before, if she has a boyfriend, stay in there I guess, but don't make any advances, and don't badmouth the boyfriend!
If she doesn't have a boyfriend, ask her to go for a coffee sometime. I don't know why, but with every girl I've spoken to, when a guy asked us to go for a coffee, we just know that they like us (It has always been the case with me...might not be with everyone) and maybe act a little more flirty so she gets the picture.
I'm a girl, btw.