Can't make out the signature, but did your friend's father have any connection to MI6 and/or Bletchley Park ? Because MI6 had a wartime SCU which included covert radio communications in Europe and North Africa. These MI6 recruits (usually soldiers with wireless operator experience, e.g. Royal Corps of Signals) wore civilian clothes, as well.
SCU3 was the Radio Security Service that intercepted enemy agent transmissions.
SCU3 which was based in Horwood, Bucks. I believe Brigadier Richard Gambier-Parry ran the SCU.
If your friend can obtain her father's WW2 service record, she might see he was possibly a Royal Signals operator, and that his record refers to “Special Enlistment”. The whole SCU operation was of the highest secrecy, and even wives were not allowed to know what their husbands were doing.