When I have had this problem in hotels, I’ve left my room and gone to a working Wifi location. I also think the guest can set up his phone to be a hotspot to connect to.
Regardless, you probably aren’t really required to compensate him since this is out of your control. However, in this rare case I would probably give the guest reparation of some sort, since an amenity I promised is missing for a significant amount of time. Maybe 1/3 off his room rate for not being able to use the internet during working hours. Or cancelling completely and refunding your portion (don’t do this yourself - contact Airbnb). You might even ask Airbnb or try yourself to find him a Wifi connection or another place to stay. Whatever feels comfortable and fair to you - there is no industry standard or best practice that I know of for something like this.