Apple — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Flesh is safe in slices. Seeds contain cyanide compounds — remove the core and seeds. Small amounts of peel are fine.
Short answer
Yes, cats can eat apple flesh with care, but remove seeds. Watch for signs of illness and call your vet if anything seems off. Offer it plain, watch for vomiting, and call your own vet if anything seems off.