If it hasn't been already stated then, I think that we should not get involved in another admins bans. There are many instances where someone was banned by an admin and they go ask another admin to deal with it. If a banned player comes up to you and asks you to deal with their ban, and you did not ban them, just tell them to go talk to the admin that banned them. The banned player needs to be responsible and mature enough to address the admin that was present in that issue because not everyone else will know exactly what happened.
The banned player should re-apply with an application so that we can have records. Also, if someone did not know already, a player will get automatically demoted if banned in-game. If you ban through sourcebans, you should demote the player using the rnd administration page. If you are not opted in for that, simply ask an admin that is so they can demote the banned player.