No due process at all, just an allegation. Now the father and stepdaughter are disarmed. How is this good? The alleged abuser will still find plenty of weapons lying around the house, and the stupid girlfriend and the father, have had their guns taken away.
Due process does not require that a conviction be obtained before any loss of liberty is permitted. Otherwise, arresting someone would be unconstituitional.
Firearms are much more useful as weapons than other devices. If you think this guy is so dangerous since he still has "plenty of weapons lying around the house," then why do you own firearms if you can find plenty of weapons lying around the house, hmm?
The issue of cohabitation - the father's firearms being taken - is slightly less clear, although the fact that the son presents a clear and present danger to society militates towards requiring certain measures - seizing the father's arms, or mandating that he store them in a manner such that the prohibited person cannot obtain them. With the right to own firearms comes the responsibility (oh that horrible word!) to ensure that unauthorized persons cannot use them inappropriately.