Many states recognize a tort of wrongful termination in violation of public policy. In many states, for example, an employee can sue for wrongful termination in violation of public policy after being fired for (1) exercising a legal right, such as voting, (2) refusing to do something illegal, such as submitting false tax returns or lying on reports the employer is required to submit to the government, or (3) reporting illegal conduct.
If he PREVENTED them from their free exercise of voting as they chose, he has problems, including, generally Civil Rights problems.
If he just told them how to vote, he is exercising HIS legal rights.
And short of going into the voting booth with them or overseeing a mail-in ballot, how could he PREVENT their free exercise of their voting rights.
I know, it’s sh**y, but not illegal, and probably not actionable.
In addition, even of their was liability, what are their MONETARY damages?
Most probably, a legal non-starter.