In my experience with mixed riding, if it's smooth desert trail, you barely need any grip at all. With pretty much any green woodland, there's always some mud or passing over roots, and you'll start needing some traction.

If your friend thinks it could be done with a cyclo-cross bike, Schwalbe has Sammy Slick in 700 x 1.3 that seems pretty nice for dry fire road use. If you suspect rolling through a small amount of mud, Schwalbe Racing Ralph might be better. For anything more difficult, I'd probably for for a 26" where you can get more serious grip.