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!
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?