Sugar/fructose tolerance does vary by horse but, even at that, the sweet treat should be minimal.
My truly insulin resistant 17 yo horse can't even smell apples, carrots or alfalfa without his insulin spiking. He founders if you just say the "F" word.
The 25 yo with Equine Metabolic Syndrome gets 1-1/2 pounds of well soaked/mushed up timothy/alfalfa cubes every day. This fella has never had so much as a laminitic event and he was diagnosed five years ago.
When the barometric pressure makes a big swing and/or we are dealing with horrible humidity, I take him off his TC Senior at night
and feed him 3/4 pound of rice bran with a big handful of carrots and apples that have been crushed in the blender.
I've done for that a couple weeks at a time and, the purpose is because he now colics at the drop of hat, has hind gut ulcers and most likely lipomas in his digestive tract. I only do that when he comes in for the night, as that seems when he's most vulnerable to colic. For him, the apples and carrots have been a huge help and haven't spiked his insulin.
Unless the horse in question is at home, to where you can keep a continual eye on her for insulin spikes, my idea of a treat is a handful of timothy pellets. That's what my IR horse gets and also my horse with oat/corn/soy allergies