Be accountable for your actions and don't be afraid of holding him accountable for his.
Look after yourself first.
Unfortunately, if you don't take action, it was be perceived as a weakened boundary and it would turn out to be enabling further abuse. I doubt he has any intention to repay, you have no realistic expectation her can/will repay? Fraud is a form of abuse. He sounds like he fits much of
Dr Joe Carver's User, Loser & Abuser articles.
I agree w JJ. If there is anyway that you would be responsible for those charges, report this "friendly fraud" to the police. If you do not, you send a message that this behavior is okay. No matter what you say, he will listen to your actions, not your words.
You can give your recommendations re him needing treatment to the courts if he is arrested. They will most likely evaluate him and take this into consideration. There are mental health workers in prison.
Don't put his life over yours.
Almost surely he has done this to others in the past. And will do to others again in the future. Clearly he doesn't stop himself, you couldn't either, maybe the legal/therapeutic system can help him stop. The only help you personally can give him at this point are consequences. Whether he will learn from it, well, that's up to him, he's an adult, he has to should responsibility or his actions.
Meanwhile, protect yourself, he sounds like he would be recurring bad news. Learn from this, don't accept this in your life. He would have to really, really prove himself to have truly change for a long, long time before you could let him back into your life.
Promises are quick and easy, change is hard and takes time. Don't let your heart be fooled into thinking a quick promise will indicate change.