Thank you both for the input!
Yes, by "getting together with" I meant sex. Apologies for the vagueness.
I did communicate my concerns about him pretty thoroughly, and she did decide to not pursue it further, though I think it was more out of concern for my desires than out of my caution about the person himself.
I remain friends with him because he's not a bad person at all, he can just be immature. His irresponsibility and dishonesty in the past mostly came from depression and fear, and I try to be forgiving and understanding about such things. He's a good friend, and I don't give up on my friends easily. I've elected to just be wary about letting him TOO close.
Sometimes I think I feel too obligated to accept things I dislike and get used to them rather than trying to keep them out of my life.
The conflict that this particular situation caused is resolved I think. I just wonder how people in the community tended to deal with stuff like this. I know I might potentially be bothered by a partner interfering with who I wanted to be close to, and I don't want to be a hypocrite about it.
Thanks again for your thoughts on things.