Married women's surnames?

I've got myself a bit confused about a very basic point. I'm making
some tree charts for a long-widowed aunt who uses her married name,
say Mary Smith. But in FTM she is of course identified by her maiden
name, say Mary Brown. How do you resolve the problem that the tree
charts will be headed 'Mary Smith', a name she and her relatives
haven't used for decades?

-- 
Terry, East Grinstead, UK