Lisa has a contract with with the store for supply of goods and services for the tiles and adhesives. The store has a duty of care to give her accurate information relating to her requirements based on the information she gives them when detailing her needs.
She too has a contract with the fixer for fitting her tiles accuratley and to professional
standards assuming that he is trained and insured appropriately.
She also has an inferred contract with the manufacturer of the adhesives that they will perform appropriately so long as instructions are followed correctly.
The point of contest here is not one of "who to blame" but one of where the Judge will be willing to award her the damages she needs to resolve the situation. I don't believe that sueing the arse out of the numpty that put pva down on a gyvlon screed hoping to get away with it and then could get two lads to work in one room and work to a line correctly is going to help her one little bit.
I suspect that threatening the reputation and good name of the larger company (supplier and manufacturer of adhesives) would pay the most dividends in the long run. Its a cruel world but its all just a game really.