bff - best friends forever " she is my bff" just said like the letter b-f-f

You can also say, "Their friendship is only skin-deep". "A skin-deep friendship". "A superficial friendship". "A superficial acquaintanceship" (not commonly used, but easily understood). That means it is not a very profound or deep friendship. It' only as deep as the skin. All these terms are slightly derogatory. That looks like it's closer to the Mandarin phrase.

Bean, Here are some idioms involving the face and the foot. "face to face" ....in person, looking in the face This is an intimate meeting but does not require friendship. - A face to face meeting. -Talk face to face. "toe to toe"....in person, facing each other, in competition or confrontation -The wrestlers stood toe to toe, staring at each other. -Meeting someone through e-mail is easy, but standing toe to toe with the other person can be a real eye-opener. "face to face and toe to toe" facing each other, very close, in direct confrontation -For men, talking to someone face-to-face, toe-to-toe implies confrontation. For women, the same positioning suggests connection. "lose face"... lose respect -The ambassador was afraid to say the wrong thing and lose face at the meeting. "step on someone's toes"... to offend a person. -The salesman stepped on some toes when he changed the schedule for the meeting.

One phrase that comes to mind is: "fair-weather friend". Literraly, a person who is a friend only in good weather. It means they are your friend when things are good, or when the friendship is profitable to them. But if you are in trouble or something negative happens, they cease to be your friend.

"thick as thieves" (they know each other very well)