Unless the service sucks, tell her to stop being such a cheap ass. If you can't tip correctly, you shouldn't be going out to eat. If you're filling up this filly, just slip the tip when you're walking out so she can't see. Having friends who are horrible tippers is embarrassing, just a pet peeve of mine. She should know better and if she is your "friend", she would respect your work and those that do it. 15-20% is the going rate these days.