Both 14C28N and D2 will be decent, D2 is harder to sharpen but will stay sharp for longer. Either steel will last longer than the joint where blade is mounted. If he really cares about toughness - he should get a fixed blade. Why would anyone pick a folder for gardening is beyond me, since they are a pain to maintain and clean in such conditions, and they generally lack toughness compared to fixed blades. And fixed blades are also generally cheaper, for obvious reasons. I'll say what others said already - Getting a Mora would be his best bet.