Here are my suggestions and a little bit on why I mention them.
25$ - Buck Hartsook Neck Knife - You can grind off the lanyard loop on the sheath and have an ultraslim knife that you could comfortably place in your wallet. It will cut fishing line, clean smaller game, whittle a spearpoint on some wood, dig out splinters etc. Mine stays in my pocket on hot, humid days and is soaked in sweat for up to 12 hours before getting rinsed and I haven't had corrosion issues. S30V with an exceptional heat treat by Paul Bos. It's a fixed blade with no scales to trap water, made of coated stainless steel. It doesn't get lower maintenance than that unless you make it out of something like titanium or H1 that won't rust. Holds an edge for a long time too. It's blade is a little shorter than you asked for, but this knife is still very capable in my opinion.
45$ - Spyderco SALT Series - I haven't used one myself, but I have used the knives their design is based on. Lightweight, rugged, capable folders that simply will not rust, period. If a lot of your camping activity consists of the fishing / kayaking side, these might make a good choice. The only maintenance they'll need is some rinsing out and sharpening once in a while.
25$ - Byrd G10 Cara Cara - These are low-priced knives with extremely rugged construction and high quality blade steel. The steel is
slightly more prone to rust than a Buck 110, but is more durable and will hold an edge much longer. Thick steel liners and G10 scales ensure plenty of handle strength. The design is extremely comfortable and fairly versatile. The choil and blade shape make for a variety of possible grips. This is one of my favorite folders.
25$ - Kershaw Storm - A robust framelock folder made out of a quality stainless steel. Features a recurved bladeshape which can be useful in certain applications. The handles might be slick around water, but the guard makes it pretty safe to use regardless. Just something a little different that you might want to consider. You can check these out at Walmart.
25$ - Victorinox Alox Farmer - I know you said he has a SAK, but to me this is THE Swiss Army Knife for the outdoors. The strong Alox handles and a very useful tool selection are the reasons why. A stout main blade, a wood saw, two screwdrivers and other useful implements. Out of every knife on my list, this is the most versatile, and aside from the Spyderco SALT is the most rust resistant.
10$ - Opinel #8, 9 or 10 - You probably won't choose this because it's high maintenance compared to the others, but it's a good knife so I'll mention it. The thin blades slice much better than many of the other knives mentioned in this thread and sharpen up easily since they're carbon steel. However, you have to watch out for rust with these, and if you don't treat the handle they can swell when wet. They're great knives for the outdoors, but if you can't be bothered to give it the extra care carbon steel demands then don't bother with it.
10$ - Moras - There are many stainless steel models to choose from. These are extremely low maintenance, being incredibly easy to sharpen due to the scandi grind and very corrosion resistant. The thin blade, like the Opinel, cuts extremely well. They're very durable knives. They're small and lightweight enough to stuff in a pocket or even wear around your neck. Personally I think one of these would make a much better choice than a folder for your intended uses.
www.ragweedforge.com has a nice selection of Moa's and Opinels, and is run by quite a gentleman. Give it a look.
