D.B.
I think there may be more at play here. He's probably not going to rehab for just weed. There may be other substances, and they may be things he uses to mask or assuage other mental health issues. If he's an addict, he's not going for 3 days. Unless he is free to walk out on his own or his parents bail him out because he complains that he hates it, he's in for 30 days probably. I think it's important that we support people who seek help, and not paint a bad or dire picture ("affect him the rest of his life") until we know more.
I would encourage her to stop trying to fix people and to let the professionals take over. I would make sure she doesn't feel so much grade pressure that she doubts herself - is that why she is attracted to guys with problems? Sometimes dating people who are "less than" in some way (irresponsible, adventurous in a bad way, directionless...) is easy - we feel competent and superior in comparison. Some people, especially girls/women, see themselves as rescuers - like it's our job to fix broken people.
The fact that she's dated 2 people with problems is what should concern you. But you can't help her by continuing to tell her that this is serious, a problem, a bad choice. That can just reinforce her feelings of insecurity. She needs to be confident going off to college so she isn't taken advantage of by other guys with forceful personalities.
I would consider getting her a life coach to help prep her for life on her own, develop the skills she will need to work with her professors and advisor, and be comfortable in her own skin. She needs to be able to be comfortable as a single woman, not relying on a boyfriend to make her feel worthy. This is beyond your ability to help her - you need to back away because you've been doing this for 17 years and some of it's not working, and also because she needs to separate from you when she goes to school. She needs a professional - but I'd get a life coach vs. a counselor. The boyfriend needs professional intervention if he's going to come back from this, and she needs to let his team work on his many issues. It's not disloyal - it's sensible.