My friend keeps going back to her lame ex

i feel like im 12 and having this problem but my friend who is even older than me has an on again off again relationship with her bf and every time i hear the same shit:

"I hate him"
"We can never be together again after this..."
"There's no way im going back"

But she does.

Dating time: 8 months. off and on like a bathroom light during that time.

I dont know how im supposed to react the next time he leaves her and she's crying about it. Am i a bad friend for being sick of the drama? ive told her to leave and she crawls back. the next time "its over" what do i do? I can't even listen to her be miserable when clearly that's what she's choosing every single time despite what everyone advised her to - leave the ass.