Without more information, it's impossible to say who Lauren is. She could be an ordinary girl, a character in a made - up story, or someone known within a particular group.
Masturbating in public is completely unacceptable behavior. It violates public decency and can make others extremely uncomfortable. In public areas, we should abide by social norms and respect the rights and feelings of others.
No, they are not appropriate to share. Masturbation is a very private matter. Sharing such stories about friends violates privacy and is also disrespectful to others.
Alexis might confront Beckett directly. She could say something like 'I think you're cheating on me. What's going on?' This is a very straightforward approach.
I don't know who Alexis is in this story without reading it. But it's likely that Alexis is either the main character or an important figure in the events that unfold in the 'creepy subway' setting.
A lawyer could be an important character. This lawyer takes on Alexis's case, believing in their innocence. The lawyer has to battle a corrupt legal system or powerful opponents who want to see Alexis locked up for good. The fanfic could show the lawyer's strategies and how they build a case in Alexis's favor.
You can be clear about your boundaries. Let your friends know that you are not interested in such topics. If someone starts to tell such a story, firmly interrupt and change the topic.
Adventure stories are also very interesting. Tales of exploring unknown places, facing challenges and overcoming difficulties can be really inspiring. For instance, stories of mountain climbers conquering high peaks.