Preventing a fraudulent registration

My parents have been targeted for what appears to be negligent/fraudulent advice by someone calling themselves a “qualified lawyer”, but having no qualifications. Yesterday they signed a form dividing their house from joint tenants to tenants in common. They don’t want to do so. How can they stop this from being registered? 

Philip - if they are concerned then they can report the attempted fraud - reportafraud@landregistry.gov.uk
You may wish to register and use our free Property Alert service so that you are notified of any significant application to update the register - Property Alert - GOV.UK