Like I say the first thing to do is to see if he can hear the relay clicking, not only will this verify the fuse and relay to be working, but confirms the problem to be after the relay, i.e. horn or wiring, more likely the former I suspect.

A neighbor had the same problem on a MK5 a few years back, which I resolved, the horn itself was the guilty culprit, I fitted a new one and all was well, maybe a common problem?

Martin