Although tachos do record vehicle speed and could be used in a speeding prosecution/defence, my understanding is that they are primarily used to ensure drivers do not exceed the allowed driving period, without the required breaks/rest.
Tachos only record vehicle speed against time, normally plotted on a disc. Your friend would be unable to prove that he was travelling at 30mph or less, at the precise time he was caught speeding from the vehicle tacho record (especially if he exceeded this speed, soon after being caught on camera).