iPhone 6s Upgrade to iOS 12.5.5

Hello! I have a neighbour, an elderly gentlemen I usually help with his iPhone, who has an old iPhone 6s.

He can't use WhatsApp anymore (he has relatives abroad), because the new version requires a newer version of the iOS he's on (iOS 11).

I've tried to help, but the only update offered is iOS 15.6, for which his phone is not compatible. He left it too late to upgrade to iOS 12, and it is no longer available.

Is there anything anyone can suggest? Many thanks!