You can do what you like..................Just dont sign anylog book or make any logbook entries........oh and dont crash, just see who and what his/her family would sue.
As a FAA CFI you cannot teach anyone anywhere for a FAA ppl without first checking that they are a US citizen. If they are not, they will need TSA, and if in the US a visa. (but you should know this).
I'm sure others will give you more advice.