Someone else reported this problem recently, and it turned out to be an issue with
Norbyte's Script Extender (a modified dxgi.dll file used by several mods, copied into the install folder).
Did you make sure your friend restored the original dll? Is he using any other mods that get placed in the game's install folder?
Does it make a difference if he creates a new profile to join from?
Are you shutting down all non-essential programs before starting the game, especially anti-virus and anything network related (utility or monitoring programs, etc)?
Have you tried hosting/joining an Arena match?
After you uninstalled the game, did you make sure the install folder was removed, or empty, before reinstalling, or install to a different location?
Did you try testing in a new game without mods enabled?
If this only happens when using mods (any copied to the install folder would need to be removed to disable), it might be faster to double check the mods themselves are identical, rather than repeatedly test new games to narrow down which mods may be at fault, or confirm any mod has an issue.
If you each use a program like DVDSig or ExactFile to scan your mods folder (..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\Mods), and swap the resulting files to verify, it would identify any differences between the folders (missing, outdated or corrupt mods). Unsubscribing and re-subscribing to mods where there are differences should presumably make sure everything is up to date, or redownloading from Nexus Mods.
DVDSig is tiny (
alternate download), portable and doesn't require an install, while
ExactFile has more features.
Alternately, you could use a program such as
Hash to create a list of checksum values to compare manually (slightly more work comparing, but no file swapping).
If applicable, disable Steam cloud support either globally (in the client click on the Steam menu and select Settings, and then Cloud) or just for this game (in the library right click Divinity: Original Sin 2 and select Properties, then switch to the Updates tab and check the Steam Cloud section).
For the GOG version, in Galaxy cloud support can also be disabled either globally (click on the GOG.com button and select Settings, and then Features) or just for this game (with the game selected, click the More button and select Settings, then near the bottom unselect the Cloud saves option).
Alternately, exit out of Steam / Galaxy, and just start the game directly from the executable when required.
Next, try browsing to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin 2 Definitive Edition' subfolder.
This folder contains the saved games, configuration files, mods and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing. With Steam/Galaxy running and cloud support enabled, the client would just download the cloud copy of your existing profile.
After that, start D:OS 2 DE, create a new profile and see if that will let you play (using Direct Connection) in a new game without mods. If that works, exit and move the mods over from the Mods folder that you wish to use, if applicable, and if it continues to work, move your existing profile back.
If the game still crashes, delete the new My Documents D:OS 2 DE folder and rename the original back again.
Try starting the game, switch to windowed mode and lower the game resolution (and maybe check the option to mute the sound volume when the game doesn't have focus), start a multiplayer game, hit Esc, open the Connectivity Menu, optionally disable online multiplayer (if it is enabled) and make sure LAN Connections is enabled (or that can be done in the options before starting a new game).
Next, start the game again from the 'EoCApp.exe' program file (doesn't matter if the first instance is started through Steam/Galaxy or the executable), select Join Online Game in the Campaign section, switch to the LAN tab, rather than Online, and try joining. With identical data and only the local network involved, that should work.
