Hi again!
We just bought a house that has renos to be completed. They almost finished a kitchen but the splashback is required.
Our building inspector mentioned the board they have used is inappropriate to go under splashback as it is not water proof - I don't know what the material is but took some photos.
It starts as the normal hardboard on the walls (in cream/yellow) and then moves into this brown sheeting.
Would it be reasonable just to put waterproofing or a Zinsser primer over the whole lot and tile a splashback?
any other suggestions without having to rip it out?

hardboard wall
moves into other unknown product
under window sash