If, as John suggests and I agree, from several hundred miles away, that the problem is " browning ", it has nothing to do with the backing, but everything to do with possible overwetting and uneven drying.
If this is the case, it should be possible to remedy, but you need to get it out of the house and checked by someone of Paul Pearces stature, in this business.
What the owner will be concerned about, is the possibility of it, not being rectified, getting worse, what your proposals are..................