You can refinance a car just like you can refinance a house. You can refi to get a lower rate, a lower payment, a shorter or longer term, you can refi and get cash out against your equity in the car, etc etc.
Credit unions will typically offer the best rates. Aside from that, most of the national lenders are about the same as far as rate, terms, credit requirements, etc. - Chase, Ally, Wells, or you can go through a place like Lending Tree where they run your app across multiple lenders to find the best deal.
If they have "credit challenges" or they're maxed out on their equity in the vehicle (100% loan-to-value or higher) they can look at subprime lenders, finance companies, etc.