The real question is, do they enjoy it? For example, I play the flute and playing brings me great joy. I play in a professional orchestra. If I were to work on my orchestral pieces on a Sunday, but felt thoroughly relaxed and at piece while doing so, I believe there would be no sin.
If they are doing it as a relaxing exercise that happens to bring them money, I see no harm. If they are doing it to earn money and it happens to relax them, they should probably wait for another day.