Not all species of tortoises and turtles are fruit eaters, so only offer it as a small part of the diet once you have confirmed that your tortoise or turtle is a fruit eater.
Non-fruit-eating species are not equipped to deal with the sugars present in fruits and can suffer intestinal problems if they eat them. In any case, the fruit of the Quince is high in malic acid, which gives it a sour or bitter taste, so it might not be palatable to tortoises.
The seeds of the Quince fruit contain hydrogen cyanide and should therefore be avoided. Do not confuse with Flowering Quince. See also Quince (Tree).