is this cheating? help

I’ve been in a very serious and commited relationship for some time with the boy I love. we’ve talked about like spending our lives together (it’s THAT serious) and today he texted me asking me if he could ask me a big question. I assumed it was gonna be something silly so ofc I fed into it and said yes. he said it was serious and wanted to call me so he did. he told me he thought he cheated on me and I flipped and asked him what he meant. he told me through the course of our relationship that he has been going to one of my close friend’s private instagram account and jacking off to videos of her twerking and in bathing suits and what not. not only is it terrible by itself, but he used to be in love with her and really really want to date her, while he also started developing feelings for me. the last time he did it was today. he told me it was the same thing as pornhub and he doesn’t get why i’m upset but to me I dont see the difference between going to my friend’s page and jacking off to her pics vs jacking off to ones some other girl sent him. I see this girl everyday. I hug her. I talk to her. she doesn’t even have any idea so I don’t feel any type of way towards her because I know it’s not her fault. but now every time we walk past her i’m going to have to wonder if he’s thinking about her. I just don’t get if you have a commited gf why you would feel the need to follow another girls private account where you know she’s posting things like that. that itself is crossing a personal line for me. but this..I don’t know how to feel rn. I called my friends and they all said they considered it emotional cheating and I should dump him but I saw a future for me and this boy and he really seems to regret it so I don’t know what to do? is this cheating?