Depending on situation and location.
I have been a security guard in my younger days, and we got involved in couple fights and wife abuse and beating once and awhile, and with an uniform it is a totally different story, as well being young and fit.
Another important factor, be sure she whants you help, and do not attack you as well, that happens quite often when you get involved in situations like this, and last do not use excessive force or loose your temper. If you are not sure you can control the situation, get back up, or try to to talk them out of the situation instead of physically get involved.