The Kona Hoss is sort of the Honda Civic of big guy mountain bikes, so it would be fine for him at his weight.

Does he want a mountain bike though? If he's going to be riding off road / fire trails, then yes, but if he's going to be doing mostly road / path riding with you to get in shape, he might be a lot happier with a road/touring bike, as they're a lot faster, and you don't get the pogo stick effect from the front suspension that an MTB would have (although that can be locked down).

If I were selling him a bike, I'd probably recommend a steel framed touring bike (something like this http://bikesdirect.com/products/windsor/tourist.htm), that'd be fast for the road, but would still be able to carry his weight comfortably.

Good luck!