I would reckon it being a Belgian, generally if I was going to clean it, which we usually do without any problems, I would first secure it along its length with ring nails every 6 - 8" before starting cleaning, water temp no more than 55 deg, reduced water pressure and clean as normal using a scrubba deluxe or similar. Do one ore two extra dry passes to get it as dry as possible and if time put the Turbo on to it.
I would only walk away if - it was in a really bad condition and would require a lot of reworking to get it clean - or if it had been badly fitted to start with.
If I have any doubts I get them to call my mate the carpet fitter, who comes out and refits and secures the carpet for them and charges them direct.
Never had a carpet of any type shrink on me, and have cleaned several BW's more than once. "Touch wood!!!"
With BW's you do not want to be getting the cotton warp on the backing wet, it is this that shrinks and causes problem. They will always go along the length and not the width but you may see some buckling along the edges.
When they do shrink it will not be along the whole length but only in a localised area, normally along traffic lines or spot marks where you have had to work harder to get it clean and the solution has got through to the back of the carpet.
Ian