from a tactical point of view like we teach about such things I would say address it and quickly move on. By doing so it lets her know you acknowledge the fact but are OK with it and won't bring it up or give her grief later about the issue. Kind of like a girl with a wedding ring. You would grab her hand, look at the ring and say, "I bet he is an accountant," (
Braddock) Then throw her hand away and don't bring it up again. That way she knows you are aware of the issue and anything you are doing after that you are OK with in spite of the fact.
This is what I would say:
YOU: Look at her wit fuck you eyes then say "So was (Your friends name) good to you?"
Her: Yah.
You: Leaning in closer to her, "Cause I wouldn't be. I don't think he spanked you enough!" Then push her away playfully and then don't talk about him anymore.
obviously you need to set it up before hand that she is a brat or that she is trouble/spoiled/etc.
I'm saying this from a tactical point of view though. Reality is how does your friend feel about the girl. Is he totally over her and wouldn't care about you dating her? Think about it first. Is it worth fucking over your friend and loosing him over this chick. I know girls are plentiful and you should never get hung up on them but use that as your rule not your excuse to be with your friends girl. In the end she is just a girl (Unless you think she is the one) and good friends are hard to find.