What to do? I hate to see this happen?

I have a friend who is actually my ex which i dated just short of 4 years. we started dating young which i know to expect change in a person and was more than flexible to accommodate that change. 2 months ago she broke up with me to figure out what to do for school and about her disease (she was diagnosed with Crohns disease). during those 2 months i kept trying to win her back to date me again, which she seemed like she wanted. a little over 2 weeks ago she told me she loved me and wanted me to buy her stuff. then on the 19th she told me she didnt love me anymore, left me with a broken heart and a lot of debt, and then on the 25th she said she was dating someone else and loved him and she had already slept with him. Over the last 2 months she started smoking and drinking (she just turned 18), which for a long time she had a problem with me drinking and i had just turned 21. Up until then she wouldve never drank or smoked. She only hangs out with her friends from work and people she knows from around work now. She has abandoned her best friends of 14 years, barely talks to them now. She tells me she wants to be friends with me then constantly talks about how she loves him more than anyone ever before, and how she is so happy and how much better he is than me and if i try to talk to her without her starting the convo i am bothering her. I am trying to be the bigger man because despite her being my ex i still love her and she is still my best friend. She and her parents dont have a great relationship, her mother is very controlling, and her dad is there but he usually doesnt do much of anything but i treated her like gold, let her do whatever she wanted (not illegal of course) and the only thing i liked to know is where she was going so that i didnt bother her and if something went wrong i could get her. She has already slept with the guy, which shes only know 2 weeks. she sleeps at his house like twice a week, her mom hates it but she wont listen to her mom. He has a kid, was a former drug addict, has his own apartment, and has a criminal record, but ive met him and he seems like a genuinely nice guy. It is like shes trying to burn down her old life, friends and all for people she barely knows. shes pretty much given up on college too. Is there anyway i can help her see that shes hurting the wrong people and only going to end up working fast food all her life or pregnant or worse if she keeps up this path. I know the person in her that i fell in love with and made good decisions is still in her because just a month ago she was telling me she loves me, and talking about having kids together someday in the future.