We have friends coming over to live,having just sold their house in the UK and purchased one here.
They are Pensioners and will need to be in the health system.
What paperwork must they legally have before applying?
If they notify their uk doctor that they are leaving there they will get a paper to take to register on the spanish system plus a euro card to use in the rest of europe including the uk. They will need an nie number and an up to date padron from their townhall.
they need to get a form S1 from the International Pensions Centre see https://www.gov.uk/guidance/healthcare-in-spain (https://www.gov.uk/guidance/healthcare-in-spain).
They should also get an EHIC from the UK (details from the same web page)
Ditto Elgin's advice. It could also be useful to ask their doctor for print-outs of their health records which they can hand to their new doctor once registered.