I think the point here is that officially its a no no, however, we all find different coping mechanisms to follow the diet.
If you look at the CD book it does say some people don't stay in Ketosis on 790 due to the milk, especially if they are carb sensative, but most people do.
Try the above options, adding a bit of shake to coffee or adding a bit of coffee to shake and then re look at it if it doesnt work.
If it is the difference between a tiny bit of milk or jacking it in then Id try the milk tbh, but, the rules are there to follow to make the diet work in the best possible way, doenst mean they cant be bent slightly but just be careful, one splash of milk can lead to two, can lead to.......
Good Luck x