Everything said is true. Perhaps a fix you might consider is to find a top notch welder who can do gas arc with stainless. It is possible, the key is, the welder must be extremely good. Then after it's welded up someone equally good at hand work with files could bring it back to the original contour. Depending on the content of the rod used, it may still show a bit if it's not a great match. I'm not sure your milling operator is all that good. Most gunsmiths that have a mill know not to take too big a cut and also not to feed it to fast or hard. That's how mills break and chatter. When I was in business, I had my own machinery. When I retired from heavy pistol work, I sold all my machinery and I found a gunsmith that I trust who is a master on the mill. When I need machine work, expecially sight work as you had done, I send it to him. I would recommend him to anyone.... "The Action Works" in Chino Valley Arizona and the owner/operator is Don Williams, a true gentleman.
PS: Under no circumstances let this hack fix that slide. He can't do it. If he could have done that you wouldn't have ever noticed. Get it back, call the loss a loss, and let someone else either repair it or salvage it.