Depending on how old they are, this could cause trouble between those two. I once caught a college coed on the rebound, meeting both her and her best friend at a bar. The next Friday, both girls came over to spend the night, the one to sleep with me and the other just in case there was trouble. They both left the next day, and then the one I slept with got back together with her boyfriend. But I also had the other girl's number and gave her a call the week after that, and we had some long phone calls that might have led to a date. But then she asked her friend if it was okay, and the friend flipped out. Even though she was back with her boyfriend, it bothered her that her best friend might sleep with me too. So her friend told me to stop calling either one of them.
Good decisions come from experience. Experience comes from bad decisions.