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There is no practical different for you. Normal milk has lactose, which is a disaccharide that is galactose and glucose (two monosaccharides) stuck together. Your body produces lactase in your digestive tract to break the bond hat holds the two mono sugars together, which is necessary for them to be absorbed. Her body does not produce that enzyme so lactose passes intact into her colon where it is munched on by bacteria (producing gas) and causes a hypertonic environment which leads to water rushing into the colon (producing diarrhea).
Lacto-free milk just has lactase added directly to the product so the lactose is broken down into galactose and glucose in the container already. It changes the taste slightly since glucose and free galactose taste sweeter as free sugars than when combined as lactose, but there’s no nutritional difference.