The solutions are not simple and quick. But could yield results over long term. I have practiced this. As an ex-copywriter I have had my writing critiqued by creative directors many times. So I state this as someone who learnt a little about written communication.

- Review material written by non-native speakers. Look for cultural anachronisms, cliches, grammatical errors etc..before all this one has to ensure if the message is functionally correct. Provide feedback. Can't emphasize how important this is. Everybody wants to be good at communicating. So tell them why you find it wrong. That would become a guideline to be followed.

- Suggest what material they should read to become proficient in using your target language. Reading good literature and comprehending it, somehow deepens ones understanding of how to use a language.

- Let them practice. More pointed feedback you give along with opportunities faster they learn. Expecting progress without this is just wishful thinking. You should not blame the other person!