When does a language partner become a language student?

Some time ago, before the italki teaching platform was introduced,  I had a very fruitful language partnership with one of my italki 'friends', in which we were both learning a lot from each other about our chosen second languages. After a time, however, my 'friend' started asking me for help with editing and correcting documents for her work. At this point, I had to point out that this was not a fair use of our 'relationship', as this was the sort of work I would normally charge companies for. I suggested that she ask her bosses to pay for an editor or send her to a proper language school to improve her English so that she could perform the duties she was asking me to do for her. I never heard from her again. I view that as a great loss, as my 'friend' was also a very good teacher.

The point of the story is that I think the main use of italki for everyone is a means of sharing knowledge, rather than making a living. However, I welcome the teaching platform that italki has now set up as a way of dealing with the problem I encountered. If I had the same problem again, I would suggest my 'friend' become my italki student, and I could not only help her to correct the particular documents she was concerned with, but also teach her more about formal business writing and other things she might need in the course of her work.
I'd be very interested to hear about experiences of other teachers that have italki students and what successes or problems they have encountered.