General voice geas is fine! If he's going to be using it on other player characters (because "general infliction of compulsion to do something" is really the "power" we're getting at here, with the status ailments/confusing being the byproduct of using it), make sure to have player permission first, and the more contrary to a monster's nature the demand is, the harder it'll be to enforce (and thus, the more dream battery it'll use to succeed).

But yes, he is welcome and encouraged to berate Captain Fantasia for its poor life choices and Forget Beam as desired. (He could also presumably buff people by shouting at them to fight on and give it their all, hard work and determination, etc, etc. but that's something else entirely, and open to your discretion.)

Approved!