Not into house plants so I looked at this site.
https://houseplantcentral.com/12-common-problems-with-monstera/

Thanks Owdboggy.

Yes, I had steered her there but none of the sites we tried were conclusive (as always!). I have suggested that she should carefully extract the plant from the pot and look at the roots since root-rot seems to be a possibility and she could at least rule that out. Or not....

I didn't like to suggest that as it smacks of criticising someone's treatment of the plant.
Overwatering and root compression are usually to blame for damage like that. We haven't grown one of them for over 40 years now. Sad, isn't it?

Root compression? Do you mean pot too full of roots? Because she has now taken it out of the pot and it is packed tight.....can't see how to add a photo here though.

We have suggested she pots into a bigger pot, loosening the roots somewhat and maybe cutting some back a bit.

My daughter loves her houseplants and says to show you this:

Your Monkey Mask Monstera enjoys weekly waterings and frequent misting. Allow the soil to dry out between waterings, especially during the winter - when you might only need to water your plant fortnightly.

I hope that helps! She also said it may not be getting enough food or light.

Check it for red spider mite as well. I lost one through that.

Many thanks for all the replies. Will pass them on to granddaughter. Sorry for the delay in acknowledging them. Got very sidetracked by having to be responsible for watering many, many pots in daughter's absence in the UK and heatwave in the Czech Republic!