Any girl who does not say yes straight away is either:
a. Not interested, and trying to think of a nice way to let you down.
b. Slightly attracted to you, and giving it consideration.
In any case, whichever way it folds out, any woman in my opinion who cannot say yes straight away and you've been friends with for a while, will not hang around for long.
Maybe she is interested but is unsure if its worth risking the friendship for. Maybe he asked her the question and said take your time and tell me the answer later?? Sometimes its easier if you dont put them on the spot and give them some time to think about it, at least now you will know it was not a spur of the moment answer and is a genuine feeling.