Lo Timbalake wrote:
Her mindset probably was that she shouldn't have to give up money she was given to finish paying your bill, because even though that would be a generous tip on a $60 tab, it's not unheard of. 
Definitely not unheard of at all but I don't see how she can feel like she has to give anything up, as it was all money given to her at the end of the night by people in the same group. He gives her $25 without a bill, not knowing what he actually owes, then another person gives $52 towards the same bill.. She ends up with enough money from the group to cover the bill plus a pretty good tip. What is there to argue about? I feel like nothing given to her was hers to pocket until the whole transaction was finalized. It's not like he had given it to her 3 hours before and was now taking it back. The OP said it all happened at the end of the night.