I don't think this is the kind of riding he is doing, though.
Honestly, when you're first starting mountain biking, the bike does not matter that much. The bike will not make the rider. What you want is a durable bike that will not break the bank. I would suggest getting an entry level hardtail (only suspension in the front) from a local shop in the $500-700 range.
Disc brakes are not going to be necessary. They look cool, but rim brakes will get the job done more than fine for what you will be doing. I really like 29ers (29 inch wheels instead of 26) but a 26er will get the job done really well, too.
When you buy a bike used, you really need to know what you're doing to make sure the bike is good/safe. When you buy from a local shop, they will do minor adjustments for free. This means when you mess up your bike, you have peace of mind because the shop will be there to help.
With this being said, here are some bikes that I recommend.
Speciazed Rockhopper (not sure if exclusively a 29er now)
-buy new from a shop
-don't worry about it having disc brakes or not
I have been mountain biking for 6 years now, I cross country race, and I recently got hired at my local shop