I wouldn’t get involved with anyone with dementia who I didn’t love with all of my heart. It’s not your responsibility. Assuming you want to just help him get home and into adult social services when he lands. That’s not going to be easy.
If you do want to help him do this I’d advise against taking him to your home. Ever. And dont expect any loans to be paid back. So don’t use your own money.
Contact his wife again maybe for starters. How was he funding his retirement? Get a photo of his passport and visa stamp. Try and dig about the financials.
I printed out some documents and sat in immigration for an hour. It was really that simple. It seems to me an agent couldn’t make it any simpler than it was.
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