J.-
I sincerely think the more you try to keep them apart the worse it's going to get for you and her. She's 18 now and can basically do what she wants, at least in her mind (whether she lives under your roof or not). I had a boyfriend at her age, who was 8 years older than me, that my parents hated. They didn't want him over at their house, didn't really want me to see him but knew they couldn't stop me.
I was so mad about the way they felt about him and were treating the situation that I moved out and got a place with him. Which lasted for all of two months before I moved back home, but I was mad enough at my parents I didn't really talk to them the entire time I was gone. My parents were right about several things in regards to him. I still loved him, it still hurt, and I was pretty mad at myself for not listening to them.
I guess my point is, she's gonna do what she wants to do anyway. You want to avoid seeing her get hurt, but that's just a part of life and she needs to see for herself what will happen. You don't want her to resent you so much she thinks moving out and in with him, or just plain staying away from you guys is the answer. She's old enough now she needs to know you trust her to make her own decisions, but also be there for her when she stumbles and falls, without rubbing it in.
You can't protect her from everything, she has to grow up and is going to make some bad decisions, as much as you are going to hate watching it. Try not to forget about yourself at 18...it's tough! Whatever happens she will learn from it and carry it on, hopefully to a better, nicer guy.