I'm assuming that b/c he's having to add fluid on a regular basis, and he might be able to get an aftermarket brand easier/cheaper, he wanted to know if it would hurt the car.

I'm not a mechanic, nor have I ever worked on a Honda steering rack, but the oem Honda fluid is the way to go from what I understand - both for the life of seals, and the rack. If you use aftermarket fluid, even the "for Honda" stuff, it will normally leak fluid faster than w/ the oem stuff, so the price savings doesn't pan out.
If you don't have a dealership near you, you could try partsgeek.com The P/S fluid costs the same as at a dealership but w/ significantly lower shipping cost.