My mom has lactose intolerance. For years, whenever she has constipation, she has drank milk to fight it. And she would always tell me to do the same when it happened to me. I tried it a few times and I did not develop diarrhea. I then read about lactose intolerance and realized that this is why my mom experiences diarrhea after having milk. I told her about it and she didn't believe me at first. I bought her lactose free milk to try and she also realized her intolerance after that.
I think it's an awful idea for a lactose intolerant person to use milk as a type of laxative. I'm glad my mom has understood that what she experienced was osmotic diarrhea.