If you can get out of the loans quickly, without turning him in, that would be best.
I'm sure you'll get advice to turn him into the police. If you hate him and want to punish him that's a good way, but otherwise, entanglements with the criminal justice system are far worse than addiction.
I would separate all finances, everything, because if you or someone else snitches, you can have all of your property forfeited. Asset forfeiture is set up to feed enforcing the Controlled Substances Act, so police are eager to steal what they can. It's their paycheck.
The worst part is, depending on what state you live in, you don't need to be convicted of a crime before drug task forces can legally kick in your door, shoot your dog and do their smash and grab and take possession of all of your stuff. ESPECIALLY with growing marijuana because the minimum sentencing guidelines are so strict. Two plants outside will suffice for a warrant. You may want to pull them up and destroy them, but DON'T throw them into the garbage.