Yes, yes, it ranges: Sometimes, you don't like the partner, but it's just a personality thing. Sometimes, the partner is actively bad for your friend. And, of course, sometimes, the partner is actively DANGEROUS to your friend. They all take different approaches. But there's no good way to deal with this, so far as I can tell, and, of course, it happens all the time because someone forgot to appoint me Grand Mistress of Relationship Approvals.
(Note: If you think this post is about you and/or your partner, it's not. It's inspired by long experience and a recent conversation with a friend who's not on lj.)