Bf follows girl in underwear

How would you feel if your bf followed a girl on insta who only posts pics in underwear/bikini/risqué poses? I asked him why and he said bc his friend said he should. It’s not like she’s an insta model, she’s his friend’s friend. I know I’m overreacting but he did flirt with other random chicks on insta before so how should I react

Update: we were together and I saw his search bar on insta and her name was at the top.

I know that there isn’t a problem with him following girls, but why look up her profile?