Chinese Internal flights are a nightmare for carrying anything liquid.
They even don't like roll on deodorant bottles, and shaving cream and toothpaste. (regardless how small, they won't let you take it on board)
Water & any other drinks also can't be taken through security check as well.
If she is bringing Yorkshire Tea (can't get that here) then tell her to not carry it on.
If she decides that she needs to buy milk, depending on which city she is in, she can buy 'Wondermilk' which is around a £1 for a pint.
San Yuan Milk is also very good, and the company has a 100 year plus history so they must be doing something right.
Another company for milk is called ba xi, which appears in a white and blue container.
I personally have san yuan, it's the cheapest, and it's the only one they do in semi skimmed variety.
The fridge section of the supermarket is where you'll find it.