My husband gives my cat a bit of sardines when he eats them, but he does so sparingly. I think as an occasional treat it should be fine, but don't replace cat food with sardines as your cat may not get all of the nutrients it needs. Talk to your vet to be sure though.
Yep, but not a lot and even less if they are the ones in oil. It'll give them very very loose poop. The ones in water are better, but still don't give him a lot. They're pretty rich and you don't want your cat's tummy upset & him leaving little piles of chewed up sardines for you to clean up.