Cousin cheating on her 4 year long bf

My cousin is cheating on her bf of 4 years

So for context, she's from a different city who's here to live with my family for her higher studies. She's 23 and in her first year of Mtech after one year of drop.

It's been a month since her college started and literally on day 2 she found a guy who's from the same city as her and has taken a two year drop after BTech. Since then she keeps telling me how similar they both are like literally even their tastes are the same.

They had a group of four with 2 guys and 2 girls and the other guy is also interested in her since the first day of college (that's what he said). The other girl got ignored as these three had a love triangle so she just quietly left their group. She didn't tell that she is in a relationship from the start. And turns out neither did they guy she's interested in who also had a relationship of 6 years.

From the last two weeks things took a sudden turn. They decided to go drinking. My sister hasn't had a drink since she was with her bf, and suddenly she chose to with these two barely after 3 weeks of knowing each other.

Then they both started going college together. Holding hands at first, flirting in front of others, and then they realise what they're doing is not gonna end well as they both have partners. So they decide to tone it down. But they couldn't do it for much longer.

They hugged each other in the college's changing area(it's a cabin with a mirror) And she even showed me the photos saying it's AI.

Then one day her shoes and jeans were murked in mud. She said she fell in college. But that was just a plain lie.

She went to a hill by bunking her college(The other guy accidentally spilled). I asked her what happened and she said that they hugged but they already did that so i assumed that things might've gone further than that.

Then yesterday, a notification popped on her phone while I put it on charge. It was Oyo booking confirmation message. Idk why but my heart skipped a beat. And I couldn't even say anything to her.

More context: Actually her bf is kinda toxic and controlling so I kinda wanted her to end it with him, but the guy she's in situationship with also has a gf whom he can't leave which is why she isn't either. But I really don't want her to date that bf of hers. He doesn't let her break up, she tried multiple times but he says he'll unalive himself if she did, so she can't do anything (Also she thinks she still loves him)