My friends (16M) girlfriend (17F) is acting suspicious. What should he do?

My friends B and F have been dating for two months. We all have a big friend group, and we call on our GC very frequently.

The other week F and I were on GC call and she sent a video of her and a guy, they were at a sleepover just the two of them, we’ll call him C.

In this video, C was very touchy, and F had her legs on him, C was touching her legs a lot, and when B saw this, he told me it looked like F and C were dating, he obviously this odd, he said that he isn't touchy with F.

B brought this up and F said she didn't notice it, he let it go but still found it odd.

Then the other day B found an insta account that was of C, on this account the bio had a quote about how he loves the sun, and he had her account tagged, he has a highlight of her and it is named “My Girl” with photos of them, some of them kissing, this is odd obviously, these are dated back to June, along with a pinned post of just them.

What makes this even weirder is that F hasn't accepted our Instagram follow request.

B has brought this up before and she said that it was an old friend group where they joked about boyfriends and girlfriends and that they just joking.

Also, we call most nights, and she's not been talkive with us.

B has said that one time one of the asked called, and she said could call soon, B said that she was still online on snap, then saw she was on Roblox with C.

To add even more, F and I both know this girl, I asked this girl about F and she said that F was sweet and that her boyfriend C. Was nice. B is now overthinking and doesn't know what to do, he wants to bring it up again but doesn't want to seem pushy or start an argument.

One of her reasons for not adding any of us is that she doesn't use Instagram that much even though we all know she does as she constantly sends reels to our GC,

B and I find it all very weird as none of us have her Instagram, so what is she hiding? Why is she very secretive, other have thought F and C are dating?

What should he do?