There are ways around the problem, but an isolator for three core is different to the one for two core, and if simply he picked up wrong reel of cable, the question is does it really need three core or is there another method?
We have seen this with borrowed lines with two way switching, and by using a wireless linked switch we can get around the problem, Quinetic switches are not cheap, so there is a balance between pull in a three core and use electronics.
In the main we worry about the borrowed neutral, in most cases actually the line is borrowed, but it means you can get current on a neutral wire which should not be there.
Using extra low voltage or in USA it is called low voltage, means the borrowed neutral does not cause danger, because USA will not conform there is a problem with the term low voltage, which most countries except for USA think as being 50 - 1000 volt AC. But since stuff is sold to both USA and other countries and the citizens of USA seem to be lacking common sense, and love to take things to court, often packaging has USA labels, in general rest of world not as pedantic as them.
Bathroom rules have it seems changed a bit, originally in a room with non opening window it was required to turn on fan without turning on the lights, this seems to have been dropped, but there is nothing to say lights need to be connected to the fan, you can have for example a pneumatic switch which when pushed runs the fan for 15 minutes.
As one point we were using two pole switches in bathrooms so we could turn the fan on independent to lights.
The PIR was another option, this ensured fan always runs when bathroom is used.
Two reasons for a fan in a bathroom, one smells, the other water vapour, but to work air also needs to enter the bathroom, I have seen vents in the door to allow air in, but it depends on the temperature of the area other side of door, using a heat recovery unit
gets around the problem of replacement air. OK £180, but although a door vent
is far cheaper, may be £9 you need some one to fit it, so looking at cost of job not cost of items used to do the job.
Be it extra low voltage, or a system which only needs twin and earth, or replace the cable, or even use a PIR or timer switch it is some thing for you to agree with your electrician.
Last house shower was in the bath, glass doors around shower, but room above and below the shower doors, so air circulated, and room filled with steam, this house shower doors seal at the bottom, so air does not circulate and room stays dry. So in this house the extractor fan is never used.
Same with mothers house, wet room and because no shower curtain or door, there was no chimney effect, so steam did not seem to fill the room, and the fan was never switched on, mainly as it sucked in cold air from the hall, and with a sliding door it was left open when room not in use, so room naturally dried.
The LABC inspector insisted on a fan, as he said even when windows could open, people could see in from the drive, so needed extractor, once the completion certificate was issued it was switched off never to be used again.
But my house did not have a fan and needed one, due to the shower doors causing the chimney effect. Never a problem with just a bath, it was the shower which caused the problem.
