Greenfield, my 2 cents for your friend: if your partner's past bothers you, don't marry them, find someone who's past doesn't bother you. Don't be afraid of being called a hypocrite, it's better to be happy and called names than to be miserable and stuck with someone you can't trust in my opinion.  If you are going to marry someone with the intention of staying with them for the rest of your life, there are no questions that are off limits - ask what you want to know about them. People shouldn't be timid when making important decisions such as marriage.

Good luck to your friend!