Noone can stop him from interviewing. Your company knows that. Just make sure they don't think you are encouraging him to do so.
IfIf the company doesn't want to offer him market salary, then the best thing to do as a friend and colleague:
Give him a great recommendation / reference.
Update, to clarify - definitely you should advocate for him getting a raise. The usual logic is that it would be costly, and also risky, to replace him. While the company could arguably hire another entry level for the current salary, there would be no guarantee they would turn out to be as productive. In practice, this logic might be more likely to be heard, if there were a specific project or customer relationship that would be damaged in a specific way in his absence. It would be hard to make that argument if your higher-ups think you provoked him seeking a raise, and thereby "created this situation".