Sugar is perfectly fine for cats, though not as a primary source of energy since it lacks everything else the cat needs except the raw energy.
However, some artificial sweeteners are considered toxic for cats. Dairy is also considered non-safe if it is not made explicitly for cats since they can not break it down properly. While kittens still have the required enzymes and gut bacteria to break down cat milk, dairy from cattle or other human friendly animals tends to be different in composition in important ways.
I do not believe you've done any permanent damage, though, since milk mostly seem to just cause some diarrhea and possible mild dehydration, which the cat should recover from given the right nutrients and access to water.
If you want to help this cat you should call a cat rescue organisation or wildlife rescue, depending on whether this is a feral house cat, or a wild cat breed. If there are none in your area and you want to help you should do some research and maybe contact a remote rescue for help finding a suitable diet for the cat in question.