Andy I think you need to start redlining this car a bit and stop being a granny :lol: they like being revved and VTECed ^_^
Your friends car problem sounds exactly like my dads car, the throttle body and iavc was cleaned and coolant drained and bled, those three seemed most likely the issue but the problem still persists, and with the spark plugs fouled, it probably is the injectors on his.
What petrol grade do you use? Have you tried running her on higher ron? They have more detergents that clean the engine and all probably better than bottled additives, and supposed to be better for older and higher mileaged cars. I use BP Ultimate and Sainsbury 97ron one, and my car seems to be more 'happy'.