Feel damn bad

Haih...
Recently i've received phone call from my ex's dad complaining to me that she's always missing in class and hang out with those bad companies.
Eventually, he just ask me to check out on her if she's in college and try to advise her back on track in studies as her exam is just 2-3weeks away...
Well, i told him honestly that his daughter and i are not in good terms anymore and we did not even say hi to each other in college..
But he was just like, just do him a favour in college..
So as i felt bad, i just promised him that i'll do my best..
Hmmm.. After thinking back, I actually felt to be responsible for her behaviour now as i guess i'm the cause of it because i guess i had her heart tremendously...
Erm,, is true that i did not intend to hurt her or what so ever, but shit just happen at times..
I dont really know how to advise her as the more i talk t0 her the more she'll be rebellious..
So i've discuss with my friends on what i should do and they were like, just stop fucking care bout her as she's old enough to know what she has been doing all the while.
If she wants to die then why you want to stop her??
I know i should just ignore her in my life but honestly, i really feel very guilty about it as if i'm not together with her then she would not have behaving like this..

Really sorry to his parents...
I really out of idea or choice so I just have to let her go like this..
Feel guilty to the max....