If a girl gives you hints that she is unavailable to you, then she is unavailable to you. The most you can do is DISPLAY your interest, take REJECTION if she is going to give it to you, and DEAL with it.
Basically, ask for her number, ask her on a date, go in for a kiss, ONLY IF THE DATE GOES WELL and you think the time is right. Any rejection at any point simply means she's not interested and that's the end of it.
But the truth is, if she mentions a "FRIEND," without you asking, then it's just a sign that she isn't interested. But the above is the only true way to find out. I mean, would you bring up a female "FRIEND" that you weren't official with if you were interested in a girl macking on you? Of course not.
And if you did ask if she was with somebody, then bad move on your part. If anything ask, "Are you single?" for a more straightforward answer. In this case, she would have answered yes, and that would be the end to it.
Ask her on the date. Work it from there. Forget her interests, forget her priorities, go with yours until you can sense her real interest. Then you can decide to back off or proceed.