My thoughts are that, yes, they broke the rules. BUT, they're older. My parents are in their 70s and don't live in the internet the way people of my generation do. I think they would have been able to put visas together for themselves if necessary, but it wouldn't have been easy (not that it is generally).
While I agree that they should have known about the rules, I feel like we get jaded because "we followed the rules." But the rules are immoral, imho, to the point it's inhumane and legal extortion. Many old people are facing financial hardship, so to expect him to pay circa $3000 for a visa plus flights is a joke and a half.
The rules are only to placate xenophobes and cut down on statistics. Not anything moral.
Someone in another group suggested that he pay for his own care and not use the NHS, but it's likely he paid into it. I have two great-aunts over 100, so I guess it is feasible that he could live for a while. But most people don't live much longer after their 90th birthday. It's not like he's the straw that's going to break the NHS' back.
*steps off soapbox*