Upvote:0
I think your first paragraph is totally correct. Also when returning, showing the Chinese passport would be correct, because that's where the residence permit is. She would want to be admitted as a resident into Japan. On top of that, Japan has suspended all visa free travel into Japan which the British passport might needed a visa. At least one of my Canadian friend did his 3 month tourist visa before he went Tokyo. If the airline ask why she has two passports, I don't think that's the airline's duty to determine if dual citizenship is allowed on a Chinese citizenship. Having the kind similar issue myself.
Upvote:3
Unless China is actually involved in the trip (if you transit via China for instance), you can safely ignore any rules about China not allowing dual citizenship.
Just show her Chinese passport with the Japanese Residence card to the check-in counter and when entering Japan. Basically, you're just doing the exact opposite of what you did on the way out.
Outbound:
Return: