I learned the hard way. He started doing drugs shortly after moving in. I tried helping him by paying for everything right away. He wrote off my car after one week... the next week he found a job and was still doing drugs. 6 weeks later, he ruined the car I bought him when he wrote off mine, lost his job and got arrested for drug possession. The next day my mother bought a plane ticket for him to come home and he left me with a $6500 debt. He never once said he was sorry or explain why he screwed up. I drove him to the airport and I honestly hope it's the last time I ever see him...