You're jealous

I call them bad weather friends, they only need you when their life is in the crapper

Don't block her. She'll come back eventually.

Nah, don’t block her. She’ll probably break up with him again and you can start over again

Don't bother with her. It's obvious that she's using you while she's alone.
Just my opinion.
Good luck

What is blocking her going to accomplish?

I don't know what good blocking would do. Not so long as you can just expect that she isn't a stable friend