Looked through his phone...EDIT

So I looked through my SO’s phone and saw that his best friend frequently sends him screenshots from tinder of girls he finds sexy (in bikinis etc.) anyways I saw he responded to one saying “fucking beautiful” how would you feel?? Because I feel like I’ve let myself go since I got with him and now I feel extra ugly ....

EDIT: back in June I found out he had been talking to 5+ other girls and telling us all the same thing, after that I told him I’d give him a second chance if he cut it off with them and he did. Ever since then we have each other’s finger prints on our phones and we have access to that. I just don’t know if it’s serious enough to be upset over or should I just message the friend and tell him not to be sending my boyfriend that kind of stuff?