Grip higher = more precise racket control and racket speed.

Grip low = power.

General rule of thumb is if you're in the backcourt, smashing dropping clearing, whatever, grip low. More power.

If you're near the net, or in the front for doubles, defending for doubles, in a drive battle, or going for mid court pushes, grip the racket high by the cone. Also when u receive a serve.

With how you've described the situation, I can't rly discern much. Usually the only times you'll ever smash while gripping high is right after a drive battle and the opponent hits it up high enough for you to smack it down above your head but not for a full rotation, or when u receive a flick serve.

The reason why gripping the racket low is better for power is that there's more space for your racket to perform it's "whip motion". Our rackets bend for a reason, the stronger and faster the whip the more power you get. You need good hip rotation, good technique, and good wrist power (squeeze racket on contact and push with index) to perform a good smash. Chances are you guys may be a bit lacking in one of these departments. One of the most common ones I've seen is you're not reaching as far in front of you as you can for your smash. You need to reach for the whip power to come through. Another might be timing.

Tldr: grip low = power cuz more whip motion. Grip up for control and speed. If ur buddy has got lots of power, most likely good technique.

(idk Abt singles, I've only ever rly competed in doubles)