Side Chick?

So recently this guy hmu and we were talking. He offered me good conversation and would say little cute things that'd make me all grinny and bubbly. Now usually when I talk to someone I investigate all their social media profiles, but a friend of mine new him so I didn't question anything. Me & this guy would text all day and night long, but he would often try to make our text convos freaky

Since I'm not in a relationship with him I would kinda dismiss it. I took it as him just wanting my goods, so when he always tried to hang out I made excuses on why I couldn't. He could tell I wasn't really feeling him but he was still very persistent. So one day I'm at my cousins house and we were bored so I tried to invite home over. Keep in mind he usually replies to me within 2 min. so when he didn't I was confused. Still I didn't think anything of it when he didn't reply until the next morning. That same morning I was scrolling through my fees and seen a picture of him and a girl... his gf🙃I thought it was funny at first because I realized he didn't reply cause he was with her. I wasn't mad because I wasn't really attracted to him from the start. But me being me, I commented and complimented how cute of a couple they were. Apparently he didn't find my gesture nice and blew up on me and attempted cussing me out like I did something wrong. And I really feel bad for his gf because they've been together for four years. Should I tell her, her man is cheating?