If your friend's floor is floorboards (not concrete) and there is an air
brick on an external wall ventilating the under floor space then she could
ask if a floor vent can be fitted instead of having a 4 inch diameter hole
bored through the wall. As far as I am aware the regulations still permit
this.

Nobodies asked so I will, what is this to do with cavity wall insulation?.
Does this replace an existing vent or do they make a new hole?

Dave