Oh boy, this is quite of a complicated situation because you are afraid of messing up a nice friendship, but remember...You already have interest in the girl...So you are just waiting for the opportunity to jump in!
Now, if she is interested in you she could have said that she gave the cute customer her # just to test you and see your reaction...! But then again, she could have actually gave him her number because she doesn't think you are gonna make a move!
So before you miss the chance of telling her what you feel...I think you should start getting a lot more flirty with the lady...And see how she responds...! If she flirts back...Then that's a good sign...Then it is about time to invite her to a movie...Or something. All by yourselves...Do not try inviting more than her..Like..(double dates...)...!!!
Now, if everything goes right...You will end up with a girlfriend I hope...And..If not...The most that can happen is that you will always be her friend..And she will not keep away from you if she already likes you as a friend...! So, before someone else takes your spot..I would get flirtier right now! :d
try secretly sending her flowers...! :d or...Buy her a rose..And put it on her car..Trapped with the wipers! :) no note..No nothing...Just to let her wonder...! :d
fun lil things like that, ....Girls like to be pampered...But remember..Not too much pampering either..So she doesnt go stepping all over you...!
:) good luck..And report back ..:d