Symptoms like are described can be caused by more or less constant stress on muscles, tendons, and joints, due to holding a "mostly fixed" position, but can also - for some of us - be caused by poor circulation to the affected joint, often from resting the "mouse elbow" continuously on a chair arm or the edge of a desk near the mouse.
The "prevention" commonly cited is the "get up and move" at regular intervals. A commonly recommended interval is "every hour," but for some a half-hour without a little stretching is probably too long. Once an injury has been created, the useful interval may drop to nearly zero, but often sufficient "resting" can be done with brief but very frequent movement, massage, or other diversions.
While exotic aids to allow you to continue to do something that hurts, with lessened (or less obvious) continued damage are (perhaps) appropriate in professional athletics, for most of us the better approach is to figure out what causes the hurt and quit doing it, or do it differently so that it doesn't hurt.
It's not often just using the mouse (or the keyboard) that causes hurt; but the continuous using in one position without frequent enough breaks to keep the body juices moving freely. And "resting the joint" may not mean putting the arm in a sling and not using it at all - although you perhaps should discuss with your "medicine man" exactly what (s)he means by "resting" it. The recommendation may be to quite mousing entirely, but unless the damage is really severe just improving the "work habit" to include better, or different and more varied, posture and very frequent (doing something entirely different) breaks may be sufficient.
If more restrictive "resting" is recommended, it certainly is appropriate to take the advice given by your professional consultant.