Can I sudo a command on behalf of another user?

sudo -u <username> <command>

sudo accepts a user parameter, which will run it as that user.

Alternatively, since you are root:

su <username> -c <command>

su aegir is what you're looking for, assuming the username is aegir. man su will give you more info on the su command.

That user likely won't be able to actually restart Apache if it's running on port 80, unless specifically configured to be allowed to do so. By default, root is the only user that can bind a service to a low port number.

You should be able to do the following

runuser -l <username> -c <command>

e.g

runuser -l Aegir -c service httpd restart

But if you are root you can just su to the user with out a password.

su -l Ageir

J