Hey, Sean. A bit of background on me: I'm a rancher in an area of mixed farming and ranching. These days I carry a Victorinox Swisstool and some type of modern folder that I can open and close one handed when I'm working and a fixed blade of some sort when I'm hunting. Today (working) it's a Benchmade 810 Contego, but the (unfortunately discontinued) ZT 0909 is the one I'd pick if I had to pick just one modern folder for work. In the past I've carried various traditional slipjoints, lockbacks, liner locks and, for a long time, a Camillus electrician's knife. Then again, I post on blade forums so obviously there's a good deal of knife nut mentality that goes into my choices so I'm atypical.
As far as what other, non knife nut farmers and ranchers in my area choose to carry:
Traditional slipjoints- Large stockman of varying makes. No other slipjoint pattern even comes remotely close in my area for popularity in farm and ranch pockets. If you're carrying something else now, you almost certainly used to carry one... and used it for everything from peeling apples to scraping gaskets.
Pliers based multitool- I see some Gerbers and the very rare Victorinox, but the Leatherman Supertool is king here. I know guys that keep more than one on hand so they can still have one on their belt if they need to send the one they normally carry in for warranty work. All in all the multitool is the "knife" that most farmers seem to carry these days.
Lockback- Buck 110 style. Mostly Bucks, but some people carry cheapie copies. It is what it is. Incidentally, it's a more popular hunting knife than work knife here, and it's still a reasonably popular work knife.
Modern folders- A fair number of guys and gals carry these, but they're mostly very cheap and very dull. Leaving those aside, the Griptillian/Mini Grip is the one I recognize most frequently, followed (rarely) by various Spydercos. One hand opening and closing and pocket clips are fantastic for "one quick cut and then put your knife away" jobs like opening bags to dump into something like a planter seed box, salt feeder, etc. It would be very, very difficult to get me to give up one handed operation on my work knife at this point.
My general recommendation is for a one hand operating folder plus a multitool if you're looking for maximum utility for hunting plus farm work if that's in the budget. A large stockman would also be a decent and classic choice. It's tough to give specific recommendations without a better idea of your budget and your friend's tastes though.