翻脸 is a separable verb (离合词) meaning to have a sudden falling-out with someone, typically used with 跟/和 to indicate the person. The splitting rule inserts aspect markers or measure words between 翻 and 脸, e.g., 翻了脸, 翻过脸, 翻了一次脸. It often implies a sharp change from friendliness to hostility.