HI. Sorry to hear about your distressing situation. I believe the hospital should have at least facilitated your husband contacting police to make a complaint, or perhaps done it on his behalf. If they havent, then you probably should, as the police should get the camera footage from the unit before its too late.
While im sure I would feel angry, disappointed and vulnerable in your situation, I think a couple of things need to be bourne in mind(in response to some of the comments below). "the hospital" is just a collection of human beings, health workers, trying (usually) to do their best. A decision would have been made by one individual (probably a psychiatry registrar) that the person who committed this assault was appropriate for an "open" ward, or Low Dependency ward. This decision would have been made based on the persons current mental state, ie calm settled, not aggressive or overly aggitated etc, combined with any known history of violence to others. However, when judging or trying to predict human behaviour, its not a precise science. Nobody has a chrystal ball.
You are well within your rights to ask for an explanation as to how/why the decision was made for this person to be on an open/LDU ward and why his/her behaviour was not able to be forseen. It is very reasonable to ask the hospital to justify their desicion making in that regard.
The other thing I would bear in mind is this. At some time, one of us may be unfortunate enough to have a loved with with mental ilness who is not in their right minds (ie does not have legal or moral capacity) and hurts someone else. People do this out of unrealistic fears/ dellusions, and are sometimes not held legally responsible. When people are acutely psychotic, they are not capable of making rational decisions based on real information. Some people in inpatients units are extremely unwell. Im assuming the person responsible has now been removed from the same ward as your husband, If not, I would certainly take that up with the Consultant Psychiatrist, and emphasise the concern you have about the potential for further traumatisation if he has to be exposed to the person who assaulted him.
I hope your partner recovers soon from this trumatic ordeal. Let the Doctors and nurses know how you are feelling and that you would like to be kept informed of how he is coping.