HOw to I advise my 21 year old daughter to avoid a relationship with a heroin addict?

My 21 year old daughter just called off her October 2007 wedding to a nice, clean cut young man who she had been with for more than two years. Good call if she was unhappy. Unfortunately, she just told me that she is now dating a 20 year old guy that was in rehab in March for heroin addiction after an overdose. He is also facing criminal charges (not sure if from same incident or another). She says "everyone makes mistakes, and he is 'clean'". However, she revealed that they drink alcohol together. I know about cross addictions, if using alcohol, NOT CLEAN! Also, she could be arrested for contributing since he is not 21 and she is. Also, he told her he stole from his friends and family to feed his addiction. When her dad and I tried to talk to her calmly she became very defensive and refused to hear anything we have to say. I am very concerned about her. More pertinent information, I am actually her step-mom; her estranged mother died in May of this year - alcohol related heart attack