I would like to know too!
I had terrible tendonitis in my left upper arm and elbow for more than a year few years ago. Nothing helped, or nothing helped in short run. But it made me miserable and I quit most sports, gained some weight but most importantly lost bunch of muscles. Since two years I am back in the game but started to develop tendinitis in the right tricep. I hope few days of rest will be enough. I hope that fast action will be enough. I ignored it in the other arm for a while and then it was just too blown up I guess. But it is just a speculation.
I had tennis elbow a couple of years ago. It takes a long time to get rid of. If he plays racket sports, it probably originate from the shoulder. I could not even lift a cup of coffee with my right hand when it was the worst. I chatted with a Spanish tennis trainer, he recommended me to train my shoulders first and foremost. And when I was capable I should do dead hang twice a day. I followed his advice and now I am 99% good.
There is no quick fix on this issue, your friend must be patent. Dead hang heals your upper body quicker than anything else.
Funny enough I noticed my tricep tendonitis while doing dead hang on gymnastic rings. I was hanging and then tried to do some left and right leg crunches and I noticed a sharp pain I know. So maybe more dead hangs?
I noticed that my shoulder joint is not moving smoothly a month or so ago and that maybe my right shoulder is harder to put flat on the ground when doing yoga before my tendonitis pain started…
NIR / Red light therapy has helped my tendon pain. In particular, my golfers elbow disappeared immediately after using my lamp. Stretching, massage, physical therapy helped but did not solve it. I also have shoulder issues that need periodic lamp work to keep healthy.