I typically fly the US and China route twice a year in both F and J. I would not worry. I guess I've been lucky that staff have always been helpful. They will get the Mandarin speaking agent to help especially if she is in the front of the plane.

As others mentioned, all flight announcements are done in English and Mandarin (although the the translation isn't word for word). Menu and movies are in both languages also.

In GF, she may not have another passenger next to her so there might not be someone who can help translate. In J there will certainly someone nearby who can speak mandarin and can help. I'm sure if she asks for help in Chinese other passengers will step up.

CBP is the one area where I agree a letter can only help.