What are the rules on that now
I remember back in the 60's - 70's when seat belts became compulsory, we had a car (HR Holden ) without them fitted, but a mechanic friend of Mum's said that we didn't need them as they hadn't been fitted originally.
Does the same thing still apply
I think your Mum's Mechanic friend is in for a rude shock.
Or at least he would be, if he was in Victoria.
Used to be that front belts had to be fitted (if none previous) when a vehicle was presented for a roadworthy for a sale and transfer.
I believe that it's all passenger positions nowadays.
In Vic. the only way you could get away with not fitting seatbelts, was to retain continuous ownership from new, and never get a R/worthy done on the car.
In the States that have annual Safety Inspections. It would be a fail to not have belts in the car when inspected, as I understand it.
The VACC had spec sheets for all sorts of vehicles, and what had to be done to satisfy the requirements for anchorages.
I'm sure the NRMA, and the other similar organizations in the rest of the country would have similar info.