Bala Shark turned black?

I had a customer come in and tell me his 7in bala shark turned black and died over the weekend. Nothing else is his tank has shown any signs of anything and his water is perfect. He had with the bala, 1 large angelfish, 2 smaller bala sharks, and then a school of cardinals, rasboras, glowlights, and von rios. Nothing else has died. He has had the large ball and angel for about a year. What could have caused it to turn black and die in a matter of days?

55 gallon. This guy has been coming and shopping my store for years now. When he says his perimeters are perfect they are perfect. Just trying to figure out what could have happen.