Chat plugin without public rooms, only private chats

Hi, I would like to use the Chat plugin for private messaging only, and only after becoming friends with a particular user (I believe the Friends plugin would suit this purpose well). I've found this thread https://premium.wpmudev.org/forums/topic/chat-plugin-can-i-remove-the-group-chat-facility which essentially asks for the same thing, but I don't know which solution was used. Can anyone shed any light on this, please?