It may be that if she thinks your a total jerk, shes worried that you might be a complete jerk to her friend and she doesn't want to see her friend hurt. Maybe it would be worth talking to her friend about why exactly she thinks your a jerk.

Also, if shes insecure, which it sounds like she probably is if she takes offense at your jokes, shes going to be worried about losing her friend. Maybe she just finds the particular jokes you make insulting, and therefore thinks your a jerk, and [insert first argument here].

Of course, she probably just has the hots for you..