Sweet flesh, serious stone
Nectarines split neatly into two answers. The soft orange flesh is a pleasant, hydrating fruit that most dogs can enjoy in small slices, with natural sugars that still argue for moderation. The stone is a different matter entirely. A nectarine stone is hard, roughly textured, and exactly the size to lodge in a smaller dog's throat or gut, and inside the kernel are naturally occurring cyanide compounds that become a concern when a dog chews and cracks the stone rather than passing it whole. That is why the countersurfing in your question matters more than the fruit itself: a dog who helped himself may have taken the stone along with the slices.
Checking what this morning's raid actually involved
Start by reconstructing the fruit bowl. Count the nectarines, look for discarded stones on the floor or in his bed, and work out whether a whole fruit disappeared, stone and all. A dog who ate slices someone left cut is in a very different position from a dog who crunched through an entire fruit. Watch him today for gagging, drooling, repeated swallowing, vomiting, appetite loss, or straining, and contact your vet promptly if any of those appear or if you know a stone was chewed into fragments. Do not wait for dramatic symptoms with a known swallowed stone; your vet would rather advise early, when options are simplest.
Preparing nectarine slices the safe way
For an intentional treat, choose a ripe nectarine, wash the skin well, and slice the flesh away from the stone in thin wedges, inspecting each piece so no fragment of stone comes with it. Offer two or three thin slices to a medium dog and one or two to a small dog, as an occasional summer treat rather than a daily fruit habit, and remove the stone straight to a bin he cannot open. Skip canned nectarines in syrup, which add sugar he does not need, and never offer the stone as a long-lasting chew, however durable it looks. Dogs new to stone fruit should start with a single slice so you can see how their stomach handles it.
Making the fruit bowl a non-issue
Ripeness changes the treat as well as the risk: a firm, under-ripe nectarine is tarter, harder on the stomach, and more likely to be gulped in chunks, while a ripe fruit slices into soft wedges a dog can manage calmly. Store nectarines for the dog in the fridge once cut, serve them the same day, and compost the stones immediately rather than leaving them in an open fruit-scrap bowl beside the sink.
Countersurfing is best solved by geography: move the fruit bowl to a shelf, a closed pantry, or the top of the fridge while stone fruits are in the house, because a dog who scored nectarine once will check that bowl daily. Teach and reward a settled place away from worktops during food preparation, and clear fallen fruit from garden trees promptly, since windfall nectarines carry the same stone in a softer, more tempting package. If he raids again despite the move, tell your vet what he took, stone included, rather than assuming the last incident predicts this one. Noting the stone-fruit rule in KinsLe through Care Teams ensures anyone house-sitting or feeding your dog knows nectarines are served sliced and supervised, never left whole within reach.