Reality check: any mention that renting the airBnB out is illegal requires the OP to be ready to move out. Because that’s what will happen if Landlord gets threatened. He knows Foreigner has virtually no pull in this situation, and there are plenty of tenants out there. He might even make more money renting Foreigner’s room out through airBnB or some other short-term rental source than with Foreigner in there long-term.
Usually the easiest way to handle this kind of thing is simply to look for another place, and then use the airBnB thing as leverage to get out of any penalty clauses in the rental contract when you do move…and of course Foreigner is not moving because of the airBnB situation, it’s because of a regrettable and horrific personal situation with an elderly family member back in Home Country.
Save face, grease the wheels.