As @Fireflybaby says there needs to be a lone worker policy. My thoughts on this are below but I’m not a lawyer or anything so I’d suggest consulting the HSE site risk assessment pages for the proper info.
The employer is legally obliged to carry out suitable and sufficient risk assessments, considering their employees and others. clearly encounters with clients dogs of any breed, but particularly those that are higher in the “attack/bite stats”, would be a known hazard. As would lone working generally, but surely also lone working + encounters with dogs within a “room or home” together as to me that increases risk than if he was, say, meeting the client in a public space.
Even with this and supporting policies and procedures, employees like your husband should have appropriate training in dealing with the hazards and risks of these (clearly often very difficult) situations, including doing their own dynamic risk assessment when arriving at a home (this is quick) and I would say walk away if the situation feels too unsafe. Again, this would be stated in the risk assessment.
If I were your husband doing this job, I would also report each visit which gave him cause for concern. presumably there is a health and safety reporting system, which you’d hope includes reporting hazards generally and near misses, as these are vital in informing the employer, and it makes sense because if they avoid these, eventually there will be an incident. But if they don’t know about it, they could say they didn’t know.
Finally there are responsibilities on employees in law also to take care of their own and others’ health and safety at work, even though the main responsibility is on the employer. So again, by following health and safety guidance from employer, reporting concerNs, looking out for self and others.
Thats my understanding anyway but he can look at the HSE pages on risk assessment to get full chapter and verse on his employer - and his - responsibilities under health and safety at work law in the uk.
I love dogs of all kinds but in the workplace I would need to follow employer protocols. It sounds from the little you say that he is doing a very difficult and important job, and he should be able to go to work knowing that all is in place to have the hazards and risks as low as is reasonably possible. I’m a bit shocked that if it’s unionised this isn’t already well controlled, although I’ve never worked in a unionised industry so can’t really comment.