My ex sister in law actually did this with a new puppy :(
It took weeks to convince them not to and I suspect they were still doing it in secret for longer
Wouldn't that possibly kill the dog though? In any case, look up a nearby biology lab, your ex-SIL's dog might be an interesting research specimen. I wonder what it does to the intestinal flora of the dog.
The other way around (humans drinking dog milk) is simply impractical because a bitch has 8 tiny nipples with very little milk in them, and they don't like being milked at all.
Well, there is a difference between the dog drinking some breast milk, and a puppy being brought up on it. Babies get their gut biome from their mothers with breastfeeding, so if it wouldn't cause a constant diarrhea and dehydration death, it could be interesting and shed light on some human medical topics even.