Do I tell his girlfriend??

So, bit of a long story - for the last month and a half I’ve been talking to this guy, he’s been over at my house at least 3 times a week and honestly he could have won an award for his acting because he had me and my friends convinced that he genuinely liked me. So last Monday he was here and told me about how he misses me when I’m not with him, that he doesn’t want to jump straight into a relationship but that he wouldn’t be going anywhere? And kept going on about how he doesn’t kiss anyone the way he kisses me and just generally being so cute. Everything was fine he went home and then 2 days ago he said we’re getting too serious and he didn’t want to get too attached to me and I kind of played it cool and said well if your getting attached and don’t want that we can leave it and then yesterday I told him how I feel and basically he said he had feelings for me but doesn’t want anything too serious so we haven’t spoke since and then on Instagram he reposted a picture of him and a girl, it’s pretty obvious that they’re both together. My friends advice was to tell her because chances are he was speaking with both of us at the same time and if it was me I’d want to know but I just think maybe I should not get involved in their relationship. What should I do?