So there's the follow list, and I think there's the contact list, though in the case for DEGA, for the friends list, a new NIP needs to be created, where an npub adds another to their friends list, but it'd appear as "one-sided" (yes I'm giggling x3) on thr client side until that other npub does the same, then the state would be "friend" upon discovering the other published event that would contain their npub.
At which point, the chat system would be the same as DMs, but with a specific tag (tags?) to know that it's coming from a friend (anything else would be ignored)
I would imagine this would also be used for a discord-like client.