Do I tell her the truth or not?

One of my best friends has been having relationship issues. He tried to dump her a couple of months back, out of the blue, saying that he was spending too much time with her when he should be studying. (He was really spending too much time at his other job and skipping classes to go there and thats why he wasn't doing well, not because he was spending time with her.) They got back together after many tears and throughout I was her shoulder to cry on. When she got back together with him, I wasn't thrilled because I could see that he was messing her around, but I was nice to him for her. Now he has tried to dump her again for the same reason. She is sitting and waiting for him to come back to her, and I sat and sympathised for awhile, but then I got into an argument with her because I told her that maybe she shouldn't take him back. Its obvious that he can't put his issues aside and he'll do the same thing again in a couple of months time. Was I right to say it? What would you have done?