Again, not professional advice by any means, but I would frame the experience similar to a job.
Rather than, "Has spent several years working with a Taiwanese friend to improve her spoken and written English to better integrate her into her career in the United States," try posing it as your job was Private English Tutor (Volunteer). Or something like that.
Your duties are Tutoring an individual in the English language, Working with the individual to proofread and edit English-language work-related documents, Served as a coach and reference on English grammar and pronunciation, etc.
Something like that?