Toilet. Cleaning Advice Appreciated.

[Sundae]

My Mum keeps our two toilets spotlessly clean.
The one thing she would never forgive or forget is if she came back to a dirty toilet when they've been away.

However the downstairs toilet has recently developed some grey stains, which she just can't shift.
She scrubbed and scrubbed at them, has bought an even more powerful toilet cleaner and is applying it directly to the stains with the scourer, but they are not shifting.

Now I can find lots of products recommended on the internet - more and more powerful ones - and advice about using nail files or pumice stones.
But what I wonder is whether this is a thinning of the enamel rather than a stain on top of it? This house was built in the 60s and Mum has lived in it since 1972. That's nearly 40 years of rigorous cleaning, wiping, bleaching - every day.

I can easily find her a powerful cleaner - just some Googling will do.
But I don't want to make things worse.

All I know is she is genuinely concerned about this. It doesn't bother me in the slightest - you can tell it's a clean toilet. But it has her nearly in tears. She is even thinking of telling her friends not to visit because she feels sick at the idea she will think she has a dirty toilet.

Anyone had any experience with toilets damaged by cleaning?
Is it even possible, or am I making problems where none exist?