First of all, make sure that the diagnosis is correct before trying to treat it. I had a bad case of tendinitus a few years ago because of poor technique, and had to learn ways to move my arms and hands so that the tension is minimized. The only way to treat tendinitus is to stop using (or abusing) the affected tendon, as it will not get better if you keep playing the piano with a technique that hurts the tendon. I found that with my tendinitus, using splints made normal things bearable, but when the tendon is severely affected, using it at all (especially with piano) is agonizing, even with splints. I'd get some professional advice from someone who specialized in occupational therapy for musicians, especially if they themselves are pianists. That way, they can show the student a better technique to use that will allow the tendon to heal and not become inflamed again.