If you load an older save (ideally one that you know worked before), or start a new game, can your friend join?

Are you both shutting down all non-essential programs (especially anti-virus) before starting the game? Firewalls have caused conflicts, and an update to either program could cause issues, or reset options, even if they were fine before. For a third party firewall, try exiting out of the program and see if that makes a difference.

Did your friend try hosting? He can start a new game, or use a save that you send him (email, Dropbox, etc). Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames' folder.