It's kinda hard to make a call on that without knowing anything about the relationship between you and him. Step parenting can be difficult, especially with a pre-teen. If he's good to you and is attempting to be good to your daughter but running into battles, then perhaps family counseling would be an alternate route. If he treats you badly, then it's probably not worth it. If he's not even attempting to form a good relationship with your daughter, that's gonna be a really personal call for you.

I second what Kira said. Having been a step-parent, I know the challenges.

Handling a rebellious preteen is one hard thing to do, and especially so when they're not biologically related. How mature is your boyfriend? Does he know how to handle and talk to kids of that age? Is he fighting with your daughter because she's behaving badly, or does he pick on her no matter what? Does he discuss your daughter's problems with you first before he confronts her? What does YOUR gut instincts tell you to do? Like Kira said, getting help from a reputable family counselor is a good way to go. A fourth party (in your case) with a trained background and a clear mind can provide suggestions to remedy the situation.

I married my husband when my daughter was only ten, luckily she has never been a problem for us, and the two of them get along well. Even if occasional discipline was required, I'm the one who did the talking, my husband usually stayed in the background unless I needed him to back me up.