That is not true. Humans drink milk for 10 000 years and their body learned 2 handle it - at least in large parts of the world. Asiens for example get sick of drinkin milk (Not all, I think around 60% or the people, not sure there) - but it's not true to say all human body "treat cow's milk as an invader". Europeans usually don't have problems with milk cause in this part of the world milk is used for quite a while - if the dude can drink 4 litre a day without havin stomach pain etc, he obvisouly doesn't have a lactose intolerance - but like i said before, it could develop though.