It's not easy telling someone you can't recommend them for a job but sometimes it's the right thing to do. About ten years ago the director of the EFL program at the university I worked at asked my opinion about a teacher that had left the school without giving proper notice to take a job in Seoul. A few years later he moved to another school back in Daejeon so I had seen him around and, since we shared many of the same friends, I saw him out socially.
For some reason, he wanted to return to his original position at my university and that was when the director casually asked me if I thought he had "matured". While I wasn't close to the guy and knew that he was prone to make bad decisions I did think he had changed somewhat and would be grateful for the opportunity so I told the director I thought he would be better this time.
Fast forward a couple of semesters and the guy took a job in another country and pulled a runner just before the exam period. To make matters worse he had a going away party so everyone knew except for the director. Long story short, I ended up losing credibility with the director twice because I stuck out my next neck for someone who didn't deserve it.
As I said, it isn't easy telling someone you can't recommend them but when you put your stamp of approval on someone you because responsible for their actions and anything negative they end up doing reflects poorly on you, so choose wisely.