What if a person has no family or no one to make medical decisions? Does a social worker assist?

Really, need more info. How are you involved with this person?

The best thing to do for a person who you think needs help is call your County Adult Protection Services. They can come out and evaluate the situation. If they find the person is no longer competent to make decisions for themselves, they will take over. The person will become a Ward of the state. The State will assign a guardian who can then make decisions for the person.

If the person is taken to a hospital, the hospital Social Worker needs to be made aware there is no family. Then the SW needs to get the State involved and set up even a temporary guardian who can make decisions for that person.

That person will eventually become a ward of the state, which will assign a guardian to manage all their affairs into the future. They will be responsible to care for and protect this person in their best interests.

Without family at all, yes, social workers often help. And without families the decisions are left in the hands of the doctors and they will proceed in the manner that they would have suggested to family if the patient is unable to respond with his or her own wishes. Do know also that the person may have already registered documents with their doctors, have made out a POLST with their doctors, or have advanced directives already documented in their records. Though I do have family I also have all of these documents.
Your question can serve as a timely reminder to those without families to make it clear what their wishes are in given circumstances, and to speak with their doctors at the very next checkup about their own wishes.