Most people here would say that she was cheating on her boyfriend if she went out with you - and to ask yourself how you would feel if she did the same thing to YOU down the road if you two started dating.
It sounds like its not even clear if they're living together, or have been exclusive for several years.
I'd probably wait for the last day, and say something like "I know you have a boyfriend and I certainly don't want to step into the middle of your current relationship. Why don't you wait to use that phone number of mine until the right time - if that time ever comes." Add a nice compliment about how much you've enjoyed meeting her and getting to know her as a person.
And don't get sucked into being "the guy who breaks them up and now has the girl who is going thru a break-up while beginning a new relationship too soon".
It never seems to end well here at least...