Social Services care of vulnerable adults. Contact them. They will try to make you help, or they will at least ask how much you can do. Say "nothing".
Do phone the hospital and make sure they know that there is no one to help her at home and that her dh has dementia.
There will probably be a Community taxi service local to you. Ours is called Trip. They have lots of retired people who taxi elderly and disabled people around at very low prices, to the shops, hospital, visits to friends, social clubs etc. Find your local one and give her their number.
Do not take anything on, nothing at all. You will find your 'duties' will expand beyond imagination and continue for a very long time.
In her head, the one meal offered has already expanded to a hot meal every day, company every day, shopping trips, hospital visits. You have to stop it now, or they'll turn you into a ft live in carer before you know it.
A lovely gesture by your generous dh, but you have to limit it hugely (or stop it completely) else it'll be a millstone.
It's sad, yes, that a kind but small offer has taken on such a large identity.