some help

So me and my ex broke up about a month ago. 2 weeks after we broke up she went to pennstate and made out with some dude. Weve still been friends with benefits up til last week. But this past friday i started talkin to this girl from my old hs bc she was at the club i work at. we got to talking and found out we both used to like eachother and now things kind of sparked back up. The only twist is she is one of my ex's best friends. So now what would you guys do? Wait out the ex in hope she changes her mind and gets back togehter wiht me, or ruin those two girls friendships? Any input would be appreciated since im a dumbass and usually do the wrong thing.