My dad is nearly 80 and survived stage 4 lymphoma last year (was described as a miracle by the consultant), he's had hospitalisations for e-coli and a prostate infection since, and had a squamous cell carcinoma removed from his head. He's very frail and his white cell count still isn't normal. He got the NHS shielding letter, as did his partner who has lupus and liver disease.
The problem is that he has dementia - undiagnosed as he won't engage with services about it. Before he was ill he was a total powerhouse - fiercely political, active, a musician and artist and writer, very active, very independent, argumentative - you get the picture. He's stubborn and difficult and in no way accepts that he's frail now and needs to slow down.
Up to now, his partner has been keeping him going by showing him the NHS letter every morning, reminding him regularly what's happening, keeping him distracted with jobs etc. But yesterday he got itchy feet and decided he was going for a walk. She tried to persuade him not to but he got quite agitated and said he desperately needed to exercise, so she said "go for a walk for an hour and come straight back". Two hours later she got a phone call from him at his flat (they live separately but he spends a lot of time at her house, he's staying with her during the shielding period) asking whether he was supposed to be coming back to her house, as if not he'd pop to the shop for some food. She asked him how he'd got to his flat and he said he'd walked into town and caught the bus 
. She picked him up, took him home and showed him the NHS letter again, he scoffed at it and said he's as fit as a fiddle.
What the hell can we do?! He's risking her life as well as his own. I live on the other side of the country, am in the vulnerable group myself and don't drive, so I can't share the load in any practical way. I'm so worried about them both