been in an abusive relationship myself, but I've "lost" a very dear friend to one (not to death but because she dumped me) and I must recommend that even when she's complaining about him, just listen and try to keep any strong feelings that come up in you in check.  There's nothing you can do, but if you're able to be present and stick it out with her, if she ever decides to leave him you can be there to support.