Acas act as independent go betweens between parties. If they are asked to convey an offer, they cannot refuse to do so. They also cannot question the amount and they do not have an opinion of the money being offered. Nothing said in conciliation is admissable at the Employment Tribunals so the Tribunals wont take a lot of notice what went on between the parties prior to a hearing - if indeed a case reaches one.
The parties would have received a letter from Acas with the name and contact number of the conciliator concerned so the voice on the phone would not be totally out of the blue. They would discuss with the parties, if needs be, the way that Tribunals award money and its up to the parties if they take notice of this and if they decide to accept any money that is being offered.
Also Kev100, were you acting for your wife i.e on record at the Employment Tribunal or just speaking on her behalf? I dont think anyone who has a duty to contact parties in a Tribunal case would liked to be called a scumbag - after all , that scumbag helped your wife reach a settlement for an awful lot of money!!!