It's exactly what happened in that chapter. They just had sex, and he catches her recording him putting on his underwear. He wrestles the phone away and realizes she was recording the whole 42 minutes. She then says she likes recording herself having sex and watching afterward. The boy outright tells her she should get consent from the other parties, and that keeping those videos is dangerous. She shows him how he looked while he was plowing her, then outright tells him she's got videos she's taken with other people. His non-virgin brain is fried the next day from what he saw.
Then the next chapter literally starts with a shot of the the screen, but censored because of how graphic it is, and she describes, in detail, how in that exact video, she was undergoing orgasm torture.
Strictly speaking, we don't know for a fact that she showed him a gangbang. But this is the same Sasha from the doujins, and she's already directly admitted to having at least one sex friend earlier in this story. Would having a gangbang in that series of videos she showed him really be that incredible? It's looking like the doujins with him in the picture are canon.