Thanks so much for all the comments and suggestions, much appreciated. I think I will have to have the difficult conversation and be as diplomatic as possible. I will also offer an intensive few days of deep clean to give her a new start and clean slate, as it where. I have my grandson every Sunday, often overnight. He always has a shower, washes hair and he has his own clothes, underwear etc at my house. More exciting toiletries are a good idea.
The problem with the pets is really there are too many in a small 2 bedroom flat, a dog and 2 cats and Mum just doesn't have the time to look after them properly, with the best intentions.
Anyway, thanks again for the advise, its not something I could discuss with anyone in real life so I do value the input.