People can and do turn their lives around all the time and that's inspiring! Good for your cousin. I have some cousins who did that too, albeit didn't go to prison, but DID get clean and stay clean and totally changed their lives for the better. Much healthier because of it.
You said your x friend doesn't have any support. Well, that might because she's burned too many bridges and that's sad, but it is likely a consequence of her choices. She used you. You got burned. How many other people has she done that with? I'm sure you're not the only one who needed to get away and stay away from her.
Her 'ex' does sound a little mixed up himself, so you're probably right, he is not the best help for her.
You know, it's hard to say in some cases, just who is going to be the most helpful person. I think it's going to be someone who will draw a hard line and hold her accountable for everything. It's likely going to take a certain amount of tough love and in reality there are not that many people who know how to do the tough love thing right. My ex BIL ended up living with his sister for awhile after he got out of jail and into recovery and she was actually good for him and didn't put up with any BS whatsoever. As far as I know he has stayed clean and sober and has really worked on his recovery and made amends the best he could. But there are some bridges that he burned that may never be re-built.