Is s/he a PhD-level faculty member? If not, I probably wouldn't use the LOR. (Academic status matters.)
As for this person's current appointment, it would look a bit strange to receive an LOR on HS letterhead. My gestalt would be to find someone else. Surely, you have other faculty with whom you have a comparable relationship. (If not, you need to start working on that. I know I had about 7-8 faculty from whom to request letters of which I got 5 and then traded them around in different combinations to maximize my interview yield.)