Take a really good picture including his face. Send picture to the local police department in case someone reports that bike stolen.

Presumably if it's actually his bike, he'll be thankful.

I challenged a guy I saw fiddling with one of the locks on a motorbike with its alarm sounding, by asking if he'd mind unlocking one of the other locks to show it was his bike. Which he promptly did - his alarmed disc brake lock had jammed.

Of course, my confidence to do so was buoyed by the fact there were two other bikers parking at the same time as me, it was in a busy public area, and I was wearing a bunch of leather and kevlar.