I honestly think that the best outdoors folder is a humble SAK. Any model from Wenger or Victorinox with asaw is fine. It can do a lot more stuff than any single blade folding knife, no matter how good it is. The extra tools also help avoid doing stupid things with the blade.
My three favourites are the Farmer, OHT and Huntsman (or their Wenger equivalent, especially the locking 85mm ones). I also use an Electrician Plus a lot, it's like a Farmer with small sheepfoot blade instead of the can opener. The saw is great, and the awl on the alox models is awesome as well. The only drawback to SAKs is edge holding, but you can touch them up very easily and the blade geometry is excellent.
I understand the appeal of knives like the ZT 560 (I've got a few ZTs and love them!), but in practical terms I really doubt that it can be more useful for outdoors activities than a OHT (I put a plain edge on mine, of course).
However, if your friend is sdt on using a single blade folding knife then ZT is definitely a good brand to look at. Any flat ground Spyderco would work well, even if the tips might seem a bit fragile, they are great for wood working (my old Mini Manix is my favourite) and tend to be lightweight. Benchmade has the Contego, Adamas, and Griptilians. If toughness is what he's after and is willing to settle for AUS8 for a blade steel, then Cold Steel should be on his list as well. There are some very utilitarian designs using the Tri-Ad lock in their catalog (American Lawman, Voyagers, etc.), rather affordable too.