Bewsh, can't say I have heard of that. On the topic if a boiler was installed before 1996 in a bedroom it's NCS. If room has changed use post 1996, then it can have a cupboard built around it with adequate ventilation and a FSD fitted. However I believe this may just reduce the classification to NCS.
Regarding classification if a boiler is AR, unless the customer complains, it goes off, on the spur, with their permission. Fill out your paperwork and notify the customer not to turn it on until rectification work has been carried out. When an ID is issued, regardless of the customers feelings, the boiler is off and supply is cut and capped. When in that situation and the customer flatly refuses and gets aggressive, I would report the job and details to my company, fill out the paperwork and if self employed notify gas safe.
i don't believe you can over classify, you have to look after your own a### it's your job, your livelihood. I would rather ID a job, safe in the knowledge I can sleep well and the customer is also going to wake up in the morning.
So with that said I would still AR the baxi Bermuda. I know your friend did not change the use of the room, but as a landlord I would demand they change it back.