A friend of mine got sky installed a couple of weeks ago and has only realised that the installers snipped the co axial aerial cable from the aerial on the roof. Is this now standard practice as terrestrial is on the way out? Do we have any sky installers in the house?

Not a Sky installer (ex cable installer), but deliberately damaging a customers own equipment would not be standard practice I suspect. You still need an aerial to be able to record say BBC1 and watch a Sky channel, or record Sky and watch BBC1 (unless you have Sky+ but even then it is your equipment, not theirs).