I do agree that if a host has food allergies and is allowing cooking, it is better to mention these to the guest in advance. @Allison_H
(Personally as the guest I would have queried the knowledge of any trainer who suggested mushroom as a main protein source in a diet as it contains such low levels of protein and there are many better vegetarian protein sources that could be used.)
I don’t thing there is anything wrong with @RebeccaF explaining the dilemma to the guest and seeing if there is something that can be done including cooking with other protein sources in the house. Unlike you, personally I wouldn’t get irritated as a guest having this discussion with the host, particularly when I knew the host was severely allergic to the product i was using.
If the guests is insistent in cooking daily with mushrooms, as the host has such high allergy levels, then it sounds like asking Airbnb to cancel the booking would be the only solution.