I'm no allergies specialist. The only tests I'm slightly familiar with are the scratch ones, which would use in this case the common allergens. There are inhalation tests which are more used for asthma patients, not usual at all (though there may be working protocols at some places, not where I work) for rhinitis.
If she's sure this is related to the workplace (she gets better at home and in other rooms at the working place) and not to some pollen in the working area, I'd have the room cleaned thoroughly by specialists, but the ideal would be knowing what's the responsible substance, just imagine if it's some component in the wall paint.
If symptoms are only nasal and they cannot determine the allergen responsible, I'd try nasal steroids and a different antihistamine which doesn't produce the sleepiness of benadryl.