a young man buys a bungalow for his mother who has dementia. He provides accommodation for two other non related people and charges them a rent. He approaches a company to provide care assistants around the clock for all three tenants. (Supported living) Does the care provider have any responsibility to its workers for fire safety i.e. safety in the workplace? if a fire did occur, and resulted in a fatality. how would the fire service view it? would they treat it as a fire in a domestic premises? or a fire in a HiMO?

I know there are a number of questions and they have all probably been asked before but it still seems unclear to me.

I would suggest you contact a solicitor specialising in fire safety like http://www.safetylawyers.co.uk/fire-safety/ for a more authoritative reply.

However I think the common areas would be subject to The Regulatory Reform (Fire Safety) Order 2005 and the Responsible Person has a duty to implement articles 8 to 22 which includes a fire risk assessment (FRA). 

The owner or management company of the building would be defined as the Responsible Person and would be required to conduct a FRA, consequently implement the significant findings.

The guidance is more difficult I think I would use Specialised Housing Guidance from the NFCC at https://www.nationalfirechiefs.org.uk/News/nfcc-launches-specialised-housing-guidance 

If it is a HMO it would be up to the local council as that is a matter for them because they enforce The Housing Act 2004  check out https://www.firesafe.org.uk/houses-in-multiple-occupation/ for more information.