I found this:
Feral Cat Project — About the Cats
Can a nursing queen be spayed? Will she be able to produce milk after surgery?
Yes and yes. We spay many lactating queens and they recover well. An experienced surgeon works easily between the mammary glands. After being spayed, the mother cat continues to produce milk as long as kittens nurse her. Their nursing action is what stimulates milk production.
The 4 month old kittens will nurse as long as mother lets them. I had one that was bigger than mom that still nursed! And the mother cat will keep producing milk as long as she is getting suckled on.