Quince — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
Cooked flesh is safe in small amounts. The seeds contain cyanogenic compounds.
Short answer
Focus on the dog that ate it. Cooked quince flesh is generally safe in small amounts, but the seeds can be harmful. Watch the exposed dog for vomiting, drooling, or lethargy over the next few hours. Keep the quince out of reach from all pets going forward, and call your vet if the dog shows any concerning signs.