No that is totally wrong. BM is the best thing when they are sick with a gastro-type bug. It keeps them hydrated, is easily digested, and comforting. Even if he vomits it up later, he'll still be getting something from it, as it's so easily digested. Water is just water, but BM is wonderful! And it'll help him fight the bug too, especially if you have been exposed and your body is making antibodies to fight it (does that make sense?).

Jazz had terrible gastro, and the dr said the only thing that kept her hydrated and out of hospital was the fact that I was breastfeeding her on damand, and to keep doing it for as long as she needed. And she was 14 months old!