Rigid fork to keep it light, disks are always nice. Avid BB5/7 are supposed to be good mechanical ones. Also remember that hydraulics for that price will probably be low-end and he’d probably be better off with top-end mechanicals.
Go for 700C wheels if he wants quite skinny tyres and will be on the road most of the time, but if he’ll be taking it along bridleways, it might be worth looking at an “Urban MTB” which is basically a more MTB-focussed hybrid, so tougher frame and forks as well as 26″ wheels which will be built tougher than road 700C wheels. Also, that gives him more choice of knobbly tyres so he can fit something like a Maxxis Crossmark that’ll roll nicely on smooth surfaces but won’t be out of it’s depth if things get a bit muddy.
Boardman Hybrids are quite good, but a bit out of your price range. however, they are very popular on the cycle to work scheme, so there may be a lot of second-hand bikes out there being sold as people want to get a new bike on the scheme.