I doubt there is anything they can do enoch other than advise. Although these days the police can push for prosecution based on the evidence of e.g. a neighbour regardless of whether the victim is prepared to make a statement/has withdrawn as statement.
But the mw would actually have to witness abuse before they could do anything. they are a) not qualified to say how the victim came by any injuries b) have no way of knowing, even if the injuries have clearly been made by a 3rd party of knowing who made them.
The victim would have had to tell them.
So all they can do is take a note of what they see / suspect and suggest to the victim that there are people they can talk to.
If H were to say to mw that she was the victim of sexual abuse and DA at Rob's hands, speak to the police and then change her mind, the prosecution could proceed on the mws evidence but I don't think they would.
Although I think they do sometimes where there is a high degree of violence (easy to prove) and the victim is too intimidated to want to be involved in the prosecution (often the case). They can go by police evidence and what neighbours have heard etc.
Changed days. One of my school friends was taken into care because the prosecution of her mothers partner failed as she was the only corroborating witness and she was deemed too young to be reliable. Her mother stayed with the partner and gave her and her db up. 
it was heart breaking as she bore so much guilt at not being believed and anger at the system and at her mother for choosing him.o er them. I now think her dm stayed because having failed to get him jailed she was in extreme danger and had given up hope of getting away without him coming after her so opted to save her kids the only way she could.