Thanks for the replies. I'm across the different ways to do it, what I was mostly hoping for is some specific makes and models people have experience with as there are a lot of options.

It currently appears to have just a mic, no piezo. I'll get him to bring it around and we'll pull it out and see if we can work out what it is exactly.

At the moment we are considering either the Fishman Matrix Infinitely or the LR Baggs iBeam Active system as replacements as they are both completely in guitar and don't require any mods to the body.

That's kind of at the upper range of what he wants to spend. We are also considering a sound hole pickup and external pre-amp. He's suggested the Boss pedal preamp as he likes the Boss stuff and it's affordable. That Tone Dexter one does look excellent, but certainly pricey and I can only find it in the states at the moment.

Specific models of Sound hole pickup that people have had good experiences with would be helpful. Also experiences of Piezo vs Pickup vs mic I'd be interested in.