I do think there's some leeway when they are very young, in a low paying job, perhaps not a full time hours if it's a zero hours contract or something, but as a general rule I do think adults should pay their own way and not ask others to subsidise them.
In this case where the daughter is 26 and has been working for years, and in fact earns more than her mother I think the rule absolutely applies.
Particularly as a house share would be double what she is paying, and would probably be far less pleasant, involving sharing kitchens and bathrooms with far more people.
A medical student in our wider family is paying over £900 a month to rent a tiny room with a family whose mortgage has gone up and considers themselves very lucky.
Primarily because they only have to share with two adults and not 8 and can use the bathroom and kitchen at a suitable time for their schedule. This was not a given in shared houses.
Also the mum of the family often does their laundry and leaves them leftovers in the fridge to microwave whereas in shared houses people used their food without asking and they sometimes had to resort to the launderette.
They also enjoy that they only need to know the schedule of two adults and two children rather than 6-8 adults, plus their assorted regujar and irregular overnight visitors in order to know when is the best time to sleep.
And that there are no Tuesday night wild parties 