My friend is being sexually harrased online

[The Joker]

No, I'm not just talking about some guy flirting with her. This guy is describing how he wants to see her make out with girls, how he wants to fuck her, describing his penis size to her, asked that once she wasn't interested in girls to call him first, et cetera et cetera.

I believe she knows this guy in real life.

What should I/she do?

[Slippy]

Gather up lots of evidence and take it to the police.

[laurita_21]

Telling the police is the best idea, if she tries to fix this herself she could be in danger.

[Nathan_B]

tell the police

[Harley Quinn]

No, I'm not just talking about some guy flirting with her. This guy is describing how he wants to see her make out with girls, how he wants to fuck her, describing his penis size to her, asked that once she wasn't interested in girls to call him first, et cetera et cetera.

I believe she knows this guy in real life.

What should I/she do?
well first being block the guy that is contacting her, if he's blocked and she stays away from him he can't describe all these things to her. After that if she still feels unsafe, tell her to tell someone that has authority, an adult, a teacher, the police, whoever and then see what happens. Like the other posters have said though, calling the police is a very good idea. Be there for her if she wants to talk to you as she may feel uncertain towards other people. Let her know you're willing to help her.