To me this is pretty simple. Hopefully you have explained to the present client what he can expect for a good trophy in the area you're hunting. It is then up to the client to make a decision to shoot or not to shoot....here's the scenario....you find a nice typical red deer for the area and you tell the client while there may be larger ones around it's up to him to shoot or not. If he doesn't shoot you keep hunting until he does or goes home without one. This pre-supposes you don't intentionally steer him away from the area you suspect the "big one" is in.

Second scenario.....you tell the present client about the potential opportunity for the "big one" and also tell him about the other client's interest. You then tell him if he passes on the big one, if you run across it, you will make it up to him if he takes a typical animal instead. This could be in the form of a reduced trophy fee for the red deer or a "free" animal he hadn't planned on taking or some other perk. If the client says he'd like to try for the big one the PH is stuck and should try his best.

If I were the PH I would opt for the first scenario as the other has the potential to back-fire if the existing client doesn't get a red deer.