3 coats of Dulux Trade emulsion should have be sufficient (with the first coat watered down by about 20%).
As above, Zinsser make a number of coatings that will help. Part of the problem with PVA is that when wet, it re-emulsifies. A cheaper option would be to apply a dilute coat of SBR (Styrene Butyl Rubber). It is similar to PVA but doesn't reactivate when wet.
Be advised that each coat of paint subsequently will take longer to dry given that none of the water in the paint is able to soak into the plaster anymore. Effectively it means that you might need to wait longer before you can apply the next coat of emulsion and that you need to make sure that one doesn't apply the paint to thick, lest it starts to slide. It isn't te end of the world though.
Just out of interest, where did your friend find the supposed decorator? Was it one of those checkatrade type sites? I find it hard to believe that he is a proper decorator.