Proof required. External damp proofing

[Scot22]

A wall has been treated with sealant to prevent penetrating damp.
The property owner is now telling us it hasn't worked. What level of proof is required to succeed in this claim ?

[dslippy]

A problem is with hygroscopic salts. These will continue to emerge from a previously damp wall for many months as it dries out. A successfully dried wall can still appear damp for a long time.

[Scot22]

Thanks DSLIPPY. You are obviously knowledgeable. The person is now saying the wall must be tanked. It is on the first floor of a Victorian random stone building converted into 12 dwellings in 1970s.
My flat is below. Will tanking affect elsewhere in the building ?
Sorry for deviating from legal, mods please forgive. Since you know about this I would value your opinion.