iWatch compatible with iPhone SE?

Beware if you have a iphone older than series 9. The newest (Sept 2023) watch has a newer chip which will not mate with older phones...it took half an hour with Apple support to confirm this, even tho the online p.r and comments board/reviewers said nothing about it!

He has iPhone SE

If it is iPhone SE 2nd or 3rd gen with iOS 16.5.1 then yes.

See -> watchOS 9 compatibility -> watchOS 9 - Apple

What iPhone SE does he have exactly? There have been 3 different models. SE First Generation from 2016. SE Second Generation from 2020, and SE Third Generation from 2022.

Depending on the model will be the watch he can pair to it.

For 2020 and 2022 models any Apple Watch Series 4 or later will work.

For a 2016 model, Series 0 through Series 7 running watchOS 8 or earlier will work.