My father has a boil on his cheek. he is supposed to shave his beard and head urgently. what can he do avoid infecting it more and spread it on head?

Skin care: I would avoid shaving if at all possible in your dad's case... See instead if he can use scissors (very carefully of course!) to trim back any hair that needs trimming. With shaving, there is a small risk of microscopic damage to the skin which could be irritating or even provide for infection to spread. I wonder if he is scheduled for a surgery of some type? Call his doctor if you have questions.