just listen to her. no advice...just listen. she needs empathy. maybe tell her about a time when your heart got broken.
(remember i have 2 older daughters than lil'monkey and an older son..who has done some heartbreaking..)

one thing that might help, is for her to realize he isn't totally who she thought him to be, or he wouldn't have had the heart to say mean things.

boys do get pressure from their buddies. it goes in cycles. if one is broke up from a girlfriend, he will sabotoge the others. girls do it too.

has he had other pressures like school or sports?

give sis a hug from me.