How to get my boyfriend to unfollow a girl I dislike?

A while ago, my boyfriend added this girl on Instagram. I’m not sure how he met her, but he did. When i asked him he said she was a girl he met at College. When i asked when/where/how, he could never really give me an answer. Eventually, he told me (like a few months later), after i asked again, that they met in the library. And she must have remembered his name and found her (this sounds so weird). Anyways, time goes on. I tell him i dislike him following her because he was suspicious about it, and then he followed her friend too, who he never met. He eventually unfollowed the girl, but still followed her friend. This girl, she clearly is interested because, when she noticed he unfollowed her, she unfollowed him, and then would add him back. She did this a few times with a few months in between. Eventually, she stopped following him all together.

My boyfriend and i got into a huge fight. And he “ended the relationship” and then went and re-followed her. I told him, when we got back together, i disliked that. He said “ok, I’ll unadd her”. Turns out he didn’t. He just said that to see if I’d use a friend’s account and check. I don’t want him to follow her! It makes me feel uncomfortable. Because, my friend follows her too. And after my boyfriend followed her, she posted to her insta story “snap me” and posted her snap.
How to get my boyfriend to unfollow a girl I dislike?
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