"it's only been 2 weeks, it's not cheating"

From the drama you're about hear, you can tell that I'm in High School.... 
A video is going around of a girl doing it with a guy in the bathroom, of course I denied to watch these videos since it's underaged sex on video... So child porn. 
It was found out who they were in the video, and I know who the girl was. She had a boyfriend, but the guy in the supposed video was obviously not her boyfriend. 
She and her boyfriend had only been dating for 2 weeks (and I'm assuming they're not saying anymore), but because no one liked her boyfriend and since it was only 2 weeks, people are saying that he deserved to get cheated on, and my sister is telling me how it isn't cheating since they've only been in a relationship for 2 weeks. 
This is obviously cheating, right?

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