What should I do about my moms friend>shes being a b****?

Ok so im 19 and me and my ex have had a rocky relationship. We went out for 3 years when I was 14 to 17 and he was 15 to 18. We basically broke up because he was doing drugs and lied to me about it.we stopped talking for a while because he was actin psycho (like he texted me when I was in cl and unable to use my phone cuase I was taking a test and then like 10 minutes later he sent me a text that said "ok I guess you don't want to talk to me anymore have a nice life") and I just couldn't deal with him acting like that so I haven't really had contact with him since. He has tried to get in touch with me but I haven;t contacted him. I am dating someone new now. Anyways getting to my moms friend lol. I have knon my moms friend Dianne since I was a baby and I go to school with her daughter. I am good friends with her daughter. Dianna added my ex to her facebook. I found this out through diannes daughter. and I was enraged.One day when Dianne was over my house I asked her to delete my ex and she said would and the reason he is on her fb was because he sent her a friends request and she didn't wanna be rude. so i put it out of my head and figure that she would delete him. Then a few weeks later my sister told me that dianne and my ex still talk on fb and that she had invited him to a party she was throwing at her house wtf?! I didn't believe my sister until she showed Diannes fb and I saw that my ex and dianne were even flirting with each other ewwww! he told her she was hot and that a lot of guys his age would basically kill to f*** her. Shes like 50 and hes 2o. Im very angry and I feel betrayed by her. I know I haven't talked to my ex but it isnt right for her to talk to him. She is married1 I feel like telling her husband but I know that that would just hurt my friend and I don't really want to break up their marriage or anything. but I don't want her talking ot my ex what should I do? Would you feel like I do in my positon? thanks for all your answers :)