Work in France but Healthcare in Belgium

A French friend of mine who has been working in Brussels for the last 5 years is going to work in Paris. As she will be coming back to Brussels frequently (almost every weekend) she would prefer to see her doctors here instead of finding new ones in Paris.

As she will not be making any Belgium contributions anymore is there a way to make these manually, or will she have to take out a private insurance like BUPA?

Anyone have any ideas? Thanks!