My mom and my friend are having ?

I found out about a month ago that a friend of mine from high school and my mom have been having for almost two years. I'm 23 and my friend is 22, and my mom's 48. We went to high school together and I've known him since we were about 17-18. I don't approve and I've told them that. They've since started dating since I found out and now I feel like they don't care about how I feel and that they flaunt it all over the house. My friend is now a total to me and talks about how he "loves coming over now" but never used to before. He's the epitome of the party frat boy and since we know a lot of the same people, I feel like he's bragging and telling any and everyone. The worst part is that he's now become fast friends with my best friend, and they hang out all the time. My best friend told me he thinks i'm over-reacting about my mom and our friend dating and that I should "chill out." Am I wrong to not approve of them dating and should I just "chill?" Anyone have thoughts? suggestions? or been in a similar situation? Any help or opinion would help. Thanks...