Its quite common at that age especially after a loss. Obviously I don't know the details of your situation, but I would suggest ...
Firstly 70 is not old
Talk to the local public health nurse and get her/him to call in.
Find out about local active retirement groups and ask them about your issue .... they have experience.
The dog idea is excellent but only if she had one in the past.
Would she take a lodger ... extra cash and company is a no brainer.
Do up her house .... she may object but it sometimes can help them move on.
Ask her for advice / help in relation to anything real or made up .... you could always describe your situation with her as being with one of your in laws and see what shes says. Many older people don't see their own problems as an issue as they feel they are making the choice to stagnate.
Always keep in touch and accept the fact that she will drive you mad at times (repeating herself and pressing your buttons) .... they all do it.