Ring him up and tell him to go on a sub floor preparation training course. Asphalt is a Dpm and don’t fail like magnesite. magnesite will crumble when damp gets stuck in it.
Put a picture on and someone may be able to help. If your floor and walls are both soaking wet i think you should have a proper damp survey done before you start damp proofing the floor. Get the surrounding drains and gutters checked. These are the worst offenders for causing damp in a house.
