Someone I know has an adult son (20 years old) with almost no motivation. Earlier this year he dropped out of his university far from home and moved back home into his parents house and enrolled at our local university due to some mental health concerns. He is almost certainly depressed and shows some symptoms of Borderline Personality Disorder, although he is still quite young for a diagnosis. Everyone else in the family attended this local university recently, and are very familiar with the professors and the curriculum. 

His first semester at this new university has just ended, and it has not gone well. During the course of the semester he has done less and less work as far as they can tell. As I mentioned, he is taking courses they are familiar with, so they know the level of rigor involved. They have had conversations with him over the course of the semester about their concern about his performance and offered to help with anything he found difficult, but he often escalates these conversations, insinuating that the parents do not know what they are talking about and have no idea how much work he is doing or that they are overestimating how much work is required. Beyond the academic, his health is an issue as well, as he eats very unhealthily and smokes a lot of marijuana. 

Finally, the semester has ended and his grades have been posted. The son had his username and password saved in the family computer and the parents have been able to log in to his account to see the results of the semester online. A long story short, he is doing extremely poorly, failing one required class and receiving a D in another required class that requires a C or better.

The parents want to confront him about his lack of effort. He is living rent-free with them at the moment and works part time, so almost all of his money must be going to marijuana. How should they go about confronting him to stop enabling his current behavior but still give him the opportunity to succeed?