I'm about 3-4 gallons of whole milk per week during winter myself, not quite as much as your friend but I know my body, I start craving milk when the days are shorter and I'm not out in the sun working or playing. I shook off covid last winter, didn't even know I had it and I've only been sick once this winter but it was some nasty little respiratory flue that got everyone I know and I was over it in 3 days when everyone else had it over a week.