What can I do as a daughter if my mom is abused by her husband?

This is a hard one. For one thing, you do not say how old you are, but the older you are, the more credible you are likely to be. Your choices, it seems to me, are (1) to discuss the situation with your mother and urge her to consult a lawyer, or at least talk to one of the agencies which advocate for battered women; (2) to notify close friends of your mother, or relatives whom you trust, and ask them to speak to your mother about seeking outside help; (3) to notify a clergy member or other trusted person who can talk to your mother; (4) notify the police. I would, in fact, do each of these things, in order. If no. 1 works, you probably do not need to go any further. Similarly if no. 2 works you won't need to go to no. 3, and so on.