I agree that the white or black 23 tooth will get chewed up but they are the only ones that will get chewed up. Do not kid yourself, your speedcal still uses a speedo gear and does not have to be the stock one.
Is the car an auto or a manual? What gear ratio does he intend to go with?
I don't recall what speedo gears a manual uses, so I can't tell you when to use a speedcal.
If he has an auto and does not plan to go with anything steeper than 3.73's, the speedo gear( purple 21 tooth ) will work fine and will not get chewed up as mentioned above. If he intends to go with more than 3.73's, a Speed-cal is what he needs.
If he intends to experiment with the rear gears, a speed-cal is a wise investment.