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If you heard reliably that the guy has a history AND HAS NOT CHANGED or is still very shaky and can easily fall back into his past then you have an obligation to get involved, however the best way to do so would be to first confirm the info and ask a Rabbi how to go about it. You dont want to be the one to do it alone without guidance.
I personally know of a very good frum girl who was redt such a guy. While they were already dating the family had some people make phone calls and they heard some really bad stuff about the guy. Baruch Hashem we were able to save her.
She stopped dating him and a short while later he got engaged to some girl and within a few weeks she divorced him. The guy was just bad news he wasnt able to grow up and put his bad stuff behind him.
So in essence I do believe someone with a past can marry someone who had no past but that is only if his or her past is totally bygones and not current or likely to come back into his life.
I know of other stories where the guy was on drugs in the past, got off drugs, promised to not use again, got married to a great girl and then bang started with the drugs again.
Unfortunately you cant be too careful these days. If you hear that he or she had a past do extremely thorough research to save yourself, your friend, or relative from a horror story.