My conundrum (it’s good to be me and have such conundrums)

I drink coffee. I like to put milk in my coffee. So does everyone else in the office. I get agitated when there is no milk to put into my coffee because a) it doesn’t taste as good and b) I know that milk in coffee helps to prevent the calcium from leaching out of your bones. Brittle bones are bad.
There is a young lady with whom I work who will pour herself a glass of milk. Every day. Keep in mind that we only buy the smallish cartons of milk. I believe that she knows I, and the ex-receptionist, frown upon this because this morning when we were both in the kitchen at the same time she made an effort to futz about endlessly, waiting for me to leave so she could pour her cup full of milk. By the end of the day I’m sure the carton will be empty, people will start bitching about there not being any milk for their coffee, the temp receptionist will come to me for more petty cash, etc.
DISCLAIMER: I realize this is all trite, but just bear with me.
The facts are these: she knows she ought not to help herself to a glass of the “coffee milk” every morning – this is evident in the furtive manner in which she does it. My company shouldn’t be providing everyone with a glass of milk every morning. My coworkers shouldn’t have to have their coffee black because one person chose to take half the milk for her personal use. The receptionist shouldn’t have to essentially grocery shop to keep the milk stock up because it’s being rapidly depleted by one person.
Given this terrible situation, which has be me overwrought and gives me night sweats, what is one supposed to do?