need some guideance

does anyone have a suggestion as to how i can get my wife to relax her picking hand

as most know my wife has dementia, she was also hit by a hit and run driver in 2019 which accelerated her dementia and also broke her Humerus on her right arm in half she now has plates and bolts in that arm, i cannot seem to get her to relax her right wrist and arm when  she strums the strings i am working with her strumming first to hit the strings cleanly, she is so tensed up that she gets this high pitched twangy sound off the base side of the dulcimer i know what she needs to do but i cannot seem to get her to understand and i do not want to frustrate her, i also know this is going to take some time because of the dementia she does seem really interested in learning to play the dulcimer though which is a big plus 

can anyone give me suggestions how to get her to relax everything when playing or just strumming the strings