Sadly in this case the probably spent £10k on conversions and decorations to make the bathroom and nobody thought (or cared) about the electrics in terms of general safety and reasonable access for future work.I can see why domestic solutions have to be a little more creative, the customer may not have the money to fully correct and a significant safety improvement will be better than no action. Just make sure you offer the full correction in writing.
Yes, ALWAYS get it in writing, even email (but make sure you save a copy just in case).In this case I'd quote for the CU to be moved to a compliant location. Get their refual in writing, email is a powerful tool, before suggesting the socket is removed and the door to the cupboard should be screwed shut, a lock will just be removed and left off, but then the CU is not easily accessible I the event of a trip.
Check with the manufacturer: if you can get them to state the equipment is unsuitable for installation in a bathroom due to humidity & condensation then you have another very good argument for C2 beyond the obvious placement close to a bath and possible ponying about as to whether the cupboard doors make it in or out of the prohibited zone.