@Anonymous You might want to revise your tone down a notch or keep on trolling as is.
"Had she exercised the same diligence in stating the ID requirement in her listing" - there has to be an obligation listed in the Airbnb agreement with the host that mentions the ID requirement as being something to be reported under the House Rules. Only then you can claim that due dilligence towards he guest was not exercised.
I gather to the best of your knowledge, there is no contract clause specifically adressing ID document requirements in the User Agreement between Airbnb and Hosts.
While your opinion is welcome, let's keep it simple and friendly. You postulate that Public Law should be mentioned and be incorporated into the House Rules - which would form a contract between the host, the guest and Airbnb. Unfortunately Public Law superseeds contract Law. And in this specific case, it could be argued that absence of reference to the appropriate public law does not grant grounds to null and void the other portion of the contract relating to terms of payments and reimbursements.
"Airbnb may well have sided with her" - to the best of my knowledge the process is ongoing, and my post referres to unnecesary, in my view, harassment Airbnb does towards the host, my friend, in order to bully through a resolution that infridges her rights.
"You can kick and scream and whine about it on your friend's behalf all you want, but the fact is that when hosts choose to list on Airbnb, they grant the company final say in matters of refunds and compensation." - this is never final and is subject to either arbitration or higher court. I would recommend any user in such situation to petition European Small Claims court conditional if Airbnb takes away the hosts' money. Should it happen in this case, it would certainly be an option.
"That's why hosts go above and beyond in their listings to disclose anything that might come up as an issue and obtain guest's agreement to it in the booking process (this also includes stuff like potential for noise, pets on property, surveillance equipment, and stairs)." - public safety - the reason why authorities require ID document details in hosts' filing can not be made subordinate to particularities attributable to a property. I think the core of our disagreement is this point. I am sure in Germany as well, Public Law relating to safety can not be listed among particularities of a property that might impede the guest to enjoy the stay, such as pets.
"Whatever the host's legal obligations are in Spain, I highly doubt that they confer upon her a legal "right" to withhold money from a guest who didn't actually stay at her property. " - in this specific case the guest had booked the property and never showed up. Subject to terms of payments and reimbursement policy the money is due, as it would be the case with any cancelled booking or a case where a guest did not show up. Surely, being the host yourself, you know this very well.
"So with that in mind, please tell me one good reason that she shouldn't add the requirement to present ID upon arrival to her House Rules and get on with it?" - this is the question you can anwer perfectly yourself.
Should there be a clause in contract documents between Airbnb and Hosts pertaining to the ID Card requirement being part of the components that form a rental contract, then surely such requirement should be listed. I count on your help to pinpoint such clause. To my knowledge there is none. Otherwise obligation to do it, beyond mentioning the official registration number of the host/property - which Airbnb requires and my friend did, simply does not exist and in fact may be illegal in Spain.