What is the right thing to do?

I would tell her just so she knows ahead of time. I mean best case its nothing and u tell her its nothing and both of u are cool. Worst case its something but at least u told her b/c otherwise she could be like why didnt u tell me when u first saw something. Since u guys are just buddies and there are not relationship ties i would tell her...hey there friend i think u gave me an STD b/c i know that u screw other people. and just a word from the wise make sure u always wrap it up with a buddies b/c like u said u are not the only one she is screwing and if she doenst make u wear a condom chances are she doesnt make anyone