My mum (90 with AD) lives on cakes & biscuits, & even more cakes & biscuits!! & the odd mini jam sponge pudding & custard or mini trifle - basically it's all sweet stuff. We're long way past the time when she would eat anything I'd consider a proper nutritious meal. They only occasional other thing she'll eat is a very tiny fish finger or chipolata sandwich. She has 2 Aymes Complete Nutrition drinks a day, prescribed by her GP. They're like little bottles of flavoured milkshake so she loves those. The GP & dietician are now happy to just let her eat what she fancies, as long as I try to make sure everything is as high a calorie count as poss - e.g. full fat milk, biscuits with a cream filling, choc chip biscuits, not the low-fat versions of things. Also they said to mix in powdered skimmed milk with food where possible (surprisingly calorific) - that's not so easy because for example if she had soup I could mix it in - but she rarely fancies soup, or other food which I could mix it with.
So with your mum - just let her eat what she fancies - as long as she's eating & drinking that's the main thing.