Hey margaux.bugante, definitely a tricky one!
One option could be to use the "Override by Adviser" (which would pull through the full signature of the adviser listed against the client (adviser name included) and then manually adding in a line that says “Courtney Y on behalf of….” before you send the email.
Or, maybe, you could remove the site/group/personal signature all together for that user and instead use 2 email templates. One email template coded with the signature for role one and another email template coded with use the signature needed for role two. Then the user can choose which email template to use. (This doesn’t work for automated emails, obviously).