Lilith said:Either that, or have the beholder telekinesis the statue of their friend in the air and say "Listen to my words, or your friend will be dust."
Hide the petrified friend, use his disintegrate eye to carve a statue in the petrified friend's likeness, and hold *that* aloft as a threat to ensure their cooperation. Even better if he actually hands over the fake statue when/if the negotiations are finally concluded.