You can act like a tough guy in the gym, stack on the plates, and do half-reps, but that's not going to get you results. When choosing your weight for the calf raise, remember that your partner is going to make the eccentric portion of the lift a lot more difficult. Don't let your ego get in the way of your gains.
As the spotter, your job is to support your partner. Just because you're not pressing the iron doesn't mean you're off duty. Since you're applying enough force during the eccentric part of a movement to make the rep challenging, you also want to keep safety in mind. Under the extra weight of the bar, your partner is likely to reach fatigue sooner.
Remember that as you go through the movements. Keep your eyes on the bar, and prevent your partner from getting pinned.