Sounds to me like she was testing to see what your reaction was. You gave her permission to do what she did, and while you may not have said it in a serious tone she still took it as an invitation to see if she could wind you up. She was just having some fun, no harm in that right?
The only thing you did wrong was to not pursue her after he got in there. She doesn't belong to you yet so there's no point getting annoyed, but you can't just let some other guy take hold of your investment. I would have talked to her again and asked who was a better kisser, then teased her about it etc. Sometimes the most negative situations are the most fun to take charge of.
