"Concern" is one thing, but people are automatically assuming abuse, sex-trafficking, etc and suggesting those involved are in enough danger that the police should check on them. That's not just concern, that's a huge amount of judgment about another adult's relationship that no one has ANY idea about.
I am not assuming anything, that is why I think the police should check on them. Assuming something would be reporting the man abuse from the get go, instead of calling the police and requesting a welfare check.
When someone appears to be under the control of another person, it is not unreasonable to wonder about the safety of the person who is being controlled.
The alternative for me would be possibly allowing someone to get away with abuse, instead of having an objective person, with the ability to legally intervene in abusive situations, assess the situation.
I personally know someone who probably never would have reported her spouse for abuse. One day, the police witnessed him preventing her from leaving their property (the neighbor called). Because of that, they were able to press charges against him for illegal imprisonment. She did not have to testify, the testimony of the officer was sufficient and the state pressed the charges, not her. Yes, he did jail time and the police called an advocate to help her get to a safe location. In this case, if a police officer arrived on the scene, assessed the situation (through observation and interviews) and determined that there was indeed abuse occurring, then they can intervene. If they determine there is no abuse occurring, they go on their way, at most providing contact information to the couple and inconveniencing them. It is not an invasion of privacy for the police to ask questions about something that is occurring in public.