Well My view is. If you really like her, ask. she is attached but she is not married.
But bear in mind:

You could very well lose her as a friend

Her bf may very well hate you and find trouble with you,

If you two do work out, can you ever trust her alone with any guy?
Ask yourself do you really want to be with her or are you just wanting to prove something???
Originally posted by kuckuc:
Hi,
I'm very confused on whether I should ask her if I have the chance to be with her... coz I do not want to waste my time waiting.
She already has a bf, but I seldom heard from her talking about her bf.
We've been seeing each other & also chatting alot.
Please advice.