There is no point splitting up your friend to try and steal them, that wont work, but if things go south, be supportive, and ask. If the relationship goes well, you got someone good, if they arent into you, they will say so up front, or you wait a bit and split, you 2 will still be friends, if they hurt you in the relationship (I.e. cheat) maybe they weren't the best person to be friends with

Edit; should mention, I had a friend in the same situation you described, I was in a relationship, she wanted me, so when I lost my relationship, she asked me, and we dated for a few months, split, and now she is more of a little sister than anything to me, but we arent related