Not to say the other advice in the thread is bad, but don't tell her how you feel straight off, otherwise you'll be seen a bit stalkerish.
What I'd do in that situation: Spend 3 days trying to get to know her better, via herself and her good friends. After those 3 days, if she has warmed up to you, then tell her how you feel, just don't go too soppy, as yet again, it'll give a bad image to her. Try act on your purer side for those 3 days. If it works, and it should, then ease her into the idea that you do have feelings for her.
If you seriously want to know more about the situation, spend a while asking around, if he deliberately smacks her in front of people (and if there is blood splatter), then try change it ASAP, and seriously, don't stuff anything up.
If you can warm her up via MSN or whatever, as it's easier to hide emotion that way, then fine, do it that way(i've done it that way, and I don't entirely rely on speaking IRL).
The way I know some of these, well not exactly "tactics" is that I have a lung problem of sorts, my breath has a continuous stench and some emphysema-like effects, but I still can take courage to talk to the girls I had feelings for. It just depends on your "trust factor."