Does anyone know who the top publishers are for medically/healthcare oriented popular books?
I think she might want to start by querying an agent with a proposal. Have her take a look at books that are (or were) already hot in the field and do the homework of finding out who the agents were. It’s the agent’s business to know which publishers to approach. Here is one of many sites that give information she might find helpful. If there’s a writer’s club in your area, it could be a very useful resource with contacts, education, and support for the process. California Writers Club is one example of an active writers’ organization.