Although I do not understand why his mobile phone does not allow him to cancel, I have an easy solution. He needs to go to a place that rents PCs by the hour, log into his account and cancel.
You can call Airbnb customer service and ask them to cancel on behalf of the guest. I've done this before. Just make sure that the communication over the Airbnb messaging is clear that the guest is asking to cancel. It will be a mutual cancelation and not penalties will apply to you. DO NOT cancel for this guest on your own. You will pay a high price.