I would definitely plead not guilty - what proof do they have that your mate was speeding? None - except a copper who 'thinks' he was speeding - That has got to be rubbish as it is open to abuse by coppers who just dont like the colour of your car/skin etc. - you could claim that the copper was misguided as they were travelling 40 in the other direction and your mate was going 40 so how do they judge when the effective passing speed was 80mph, I would have thought a lot of people would have trouble distingishing between 80 and 70 mph, as you say has he had specific speed training and when was the last time he had his brain calibrated
You could mention in the defense that the speed camera didnt go off so how could he be speeding - although I would say at least half the Gatso cameras about do not work (get a snooper to check), although what does not work today may work tomorrow
I think they are just trying it on to bully your mate into accepting the fixed penalty - go to court I am sure it will never make it there or at least the judge will laugh at the coppers and throw it out