How to change random encounters?

So, basically this is my situation: The main character has a rival, who I would like to appear in random encounters. Easy enough. However, at a later point, the rival joins the party. It would be a little silly if he still showed up as an enemy, so at that point I would want him off the list of troops. Is there any way to do this, maybe through variables or common events? Or do I just have to make a copy of the map without him on the list of troops when he joins the party?

Any help is appreciated. (:

MA wrote something that you might be able to use.

Here: Advanced Areas Script

I don't know how you have your troops set up, but instead of placing the troops for a map on the map itself, create an area on that map that encompasses the ENTIRE map. (So it effectively is the exact same thing as making troops on the map like normal.) HOWEVER! With this script, you can set a switch so that after an event happens (like your rival joining your team) then you can set new troops in the area. Basically you'd use the same area minus your rival in the line up. Understand? It's really pretty simple and I'm sure MA would help you out.