When someone starts to drag you down with their negativity emotionally, should you keep a distance?

My ex and I dated for 5 years. It was a crazy ride. It was never really stable, with really high highs and really low lows. We loved each other though. We split up because he cheated on my with my best friend a year and a half ago. We started talking as friends 5 months ago. I went from having all A's and being attentive in school to having 3 A's, 2 B's and a F. It was hard to concentrate and I felt like there was no need to do school work. I got behind in school. Almost a week ago, I told him that I would call in in a couple of weeks because I was too confused. Now, I feel great, as I did after we split up. He's so negative and it makes me want to be the same. He smokes, drinks, and parties constantly. he has no dreams or goals and he wants to have fun. I caught up on my school work and I feel much happier. I finished the school year today with 5 A's and a D. I want to be friends with him, but if I talk to him more than once every two weeks, I start being depressed just as he is. When someone starts to drag you down with their negativity emotionally, should you keep a distance? Should you try to help them? I want to know what is going on in his life but I don't want to be affected by his issues, either.