Is it ever appropriate to correct a non-native teacher's English?
It depends on how much interest she has expressed in being corrected or trying to improve and how close your relationship with her is.  Any corrections should be generally mentioned to her respectfully and in some form of one-on-one conversation.

If she has asked in class for mistakes to be pointed out, then I would not hesitate to offer the occasional correction.  Frequent corrections are probably not welcome.

If she has not expressed interest in being corrected, I would only mention corrections if the mistake could potentially lead to misunderstandings either in message tone or content.

A gray area might be if the mistake is in formal communication (e.g. grant applications) and you work closely with her.  In that case, use your own judgement.