Apple Watch with fall detection compatible with iPhone SE

Fall Detection is available for the Apple Watch Series 4 and newer:

Use Fall Detection with Apple Watch - Apple Support

But, you would have to choose an Apple Watch, which does not have watchOS 26 preinstalled, which all new watches will have, because the iPhone has to be able to run iOS 26 as well.

So in your case, either the iPhone is an SE 2nd Gen, or your Dad will need an iPhone 11 or newer.

Or buy a used or refurbished Apple Watch 4 or Series 5, which can be paired with an iPhone having iOS 18 installed.

iPhone models compatible with iOS 26 – Apple Support

It's compatible with the Apple Watch 4th Gen and later. But the 6th Gen and later are actually still supported by Apple. And Apple currently only sells the SE 3rd gen, Series 10, and Ultra 3 which all support it.

Both the 2nd and 3rd gen SE which runs 18 also supports 26.

Correct, and iPhone SE, running iOS 18, must be at least a 2nd gen iPhone SE, which can install iOS 26.

So I stand corrected, no problems on the iPhone side 🙂