Does he use a strap?  I have longer legs  than upper body and placing my strap tightly around my legs(just behind the knee bend-tightened for no motion in the dulcimer) works way better than under my behind!

A taller seat/stool or standing with the MD on some kind of stand?

I help out with a beginner's jam - a tall gentlemen was having trouble finding a good position for his dulcimer, so he could strum without hitting the strings or holding his arm up high.. 

Suggestions: stretching his leg out to lower it, having most of the dulcimer on a sharp angle with little resting on his right leg (so he is strumming toward the end), loosen his strap & out from his body some,  sitting up straight, etc. 

I know he will find a position that works but does anyone have any suggestions that he could try?