Why would he get a mountain bike, stop suggesting hardrocks etc. They will be useless.
Search Specialized Crosstrail and choose your spec. MTB stye frame (clearance, mudguard space, rack mounts etc) with 700c wheels. Will roll nicely on the road, unlock the fork and you've got a bike that will more than fine on bumpy terrain. They range from £450 upwards. £600 might get you one secondhand with disc brakes etc.
Whyte also do the Caledonian which is a higher spec of this, triple butter frame etc but price jumps alot and I don't think you would find one second hand.
The bigger wheel size will make a massive difference, especially over the distance your talking about. Slick will help but combine 700c with slick, sorted.
Crosstrail. prefect. Could go higher spec again too.
I missed the jump part. I think you needs are too far apart. Something suitable for road won't jump well. Something that Jumps well will be crap on the road.