Users have already told you it is the managers right to choose whom he wants to add in the campaign.

Obviously the more active a user is, the better he would be for a campaign and a company. Hiring inactive users is generally not a great idea at all. I won't speculate that that means it is his alts or anything. I would just advise all managers to look at user activity and post quality when choosing participants.

If you have proof he is enrolling alts, that would be a different story altogether. Without proof you probably shouldn't post speculation, defamation isn't really fair to the manager.