crackerjack - I sense your moral dilemma and have to say that I think you are profoundly wrong to advise your friend to not select Mode "C" when carrying out his manouevres.
As I am sure you know TCAS is a last gasp safety net and despite your assurances that your friend is "a good bloke who won`t bust airspace" the road to hell is paved with good intentions as they say.
On the day when it transpires that your mate has feet of clay like the rest of us and makes a mistake during one of his zoom climbs or whatever, the fact that no Mode "C" is available will rob the other pilot`s TCAS of the ability to salvage the situation.
DoB