Water injection may keep your EGT's from climbing, add meth though, and that reduction is reduced or in some instances, nullified. Methanol is a fuel and it is injected, somewhat uncontrolled, (only by nozzle size) and no one really knows or can tell you a "failsafe" mixture for the methanol(20/80. 30/70 50/50?) or exactly when is the best boost pressure to inject it at. It should be around 15-20 psi for diesels. But by the time you reach 15-20 psi, the EGT's are already up there. Water alone,could keep them from climbing higher.
Propane will not lower EGT's a great deal either only about 50-100 degrees, if your not dumping a ton of propane. For towing power, which is what you asked about, propane is hard to beat, especially in the mountains and can be shut off completely when the extra power is not needed.
Nitrous can not be used for towing power. It is good for short bursts of power for racing , dyno runs, etc. You'd quickly go broke if you tried as a bottle doesn't last long at the track, let alone climbing a steep grade.
Personally, I would tell him to break that engine in a bit, make sure nothing is wrong or rears it's ugly head, before adding anything that adds torque and horsepower.
BTW, don't believe everything you hear