Making a GUI that allows you to unban a player and add a variable to that player

so im making a system for my mod where you have lives and i want to make a GUI that allows another player to unban you after you've run out of lives and add a certain amount of lives to the player that's being unbanned. I've made the GUI with the textfield, and i have it able to unban the player, and i've made the lives variable, I just don't know how to add lives to the player after they've been unbanned. Any help at all or suggestions with this would be great, thanks.