Mamma no longer should have milk, they aren't nursing they are just comfort sucking...she'll bat them away when she's had enough of their pestering..right now she must be still enjoying the sensation
The guy didn't really tell you the real story.
Even if she is still producing milk, if she is ailing it is not because the kittens are draining her. If you see her as being ill, take her to the vet. If you think she is just getting thin, feed her kitten-formula food, it is very rich and will build her up.
At the kittens age, I agree with Opus. But I also agree with Anniebrooke and Pip in feeding Mama the kitten food. It is hard to say what is going on exactly.
Also, it would be a good idea to separate the Mama from the kittens, that will do the trick!
Meanwhile, please take Mama to the vet as it seems from your description, she is quite ill.