Hands alone are not the only areas to address of course with anyone who has to use them so much. You also want to work the forearms throughly, all the way up into the neck, shoulders, scapula regions, and into the mid & upper level thoracics.
Two of the books aimed at MT's may be of help: Hand Maintenance Guide (For Massage Therapists) by Shogo Mochizuki,
Save Your Hands by Lauriann Greene and Robert A. Greene
Don't forget their body posture when considering what to do for clients like yours: Look at the neck and shoulder region. You will see that many people carry the head in a forward posture and may well have started to develop a slight hump from T1 into the other upper thoracics. Along with that observe the shoulders from the front, make note of any tendency toward the shoulders rounding forward.
Ask your client to assume the posture they are normally in while performing regular tasks, observe the position they are in and ask them to become aware of it themselves.
Most of the musicians & artists I've worked with in the past really need to work with shoulder & neck stretches as well as doing a variety of other stretches for the back & hips. Keep in mind many of them sit or stay in the same basic position for extended periods while holding the hands in contracted ways as well. They need to learn to take some break time, to do something else for awhile to free up the tension created by their occupations & hobbies, just like the rest of us.
I've also shown many clients how to use their opposite arm to work on the forearm of the dominant hand, utilizing the heel of the hand or the forearm & the elbow to do some deep pressure techniques for themselves. That way they can help themselves between sessions.
Hope this helps ! Good Luck with your client.