Are you entirely sure that she has no family or friends? Absolutely none?
Talk to your neighbours again about who she has in her social circle, who can help her out, take her out etc.
You have a reasonable opening line about ‘Now that lockdown is over, we are all getting back to normal…’
Maybe ‘ Oh my, we’ve all become so used to being at home, it’s time for me to get out & about again. So what are you going to be doing…?’
Explain & insist that you cannot do as much & that it is OK to get in some home help, or get out more socially.
Although late 70s is not ancient or incapable, any existing social isolation has become more deeply entrenched for many. She may be naturally solitary, less firm & agile, or just lonely - this could any of us anytime soon.
You cannot be her carer, but you may help her get in contact with people who can help. AgeUK are a good place to ask about home support & befriending services, encourage her to make contact.
U3A is lively & offers interest based activities, run for people over x years by peers.
‘Hey, have you heard of this lot U3A, loads of interesting stuff going on, what interests you? ‘
University of the Third Age: www.u3a.org.uk/
You do need extricate yourself from this possibly dependent relationship with kindness.
If you cannot this through friendly discussion, then approach AgeUK for advice & possibly revisit that call to local Adult Social Services with an absolute insistence that you will not support her.
Ideally, she’d respond to your gentle direct approach.