I think she might have to measure herself using online guides and buy online from one of the specialist sites for larger ladies. The shops only stock things that will sell quickly, hence the lack of suitable bras you’ve found.
Thank you for your suggestions. Actually, it shows how long ago it is since I visited my friend in Norwich as the shop that I think of as "Bonds" is actually John Lewis now. She tried there, but no luck.
Because of new allergy reasons, a friend needs to buy a cotton bra in person (it needs to be fitted properly as she is well-endowed) and has tried Marks & Spencers, Bravissimo and Bonds, but they all say they do not stock cotton bras for large sizes. Does anyone have a tip for where she could try in the Norwich area?