Are you hosting, or your friend?
Do you have any problem if you try the same fight in single player? If you are not hosting, you would need to get your friend to send you the save.
Please email supportdos2@larian.com, with a description (or link to this topic) and a save where this is happening (or a Dropbox or Google drive link, etc).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2\PlayerProfiles\<ProfileName>\Savegames' folder. In Windows Explorer you can zip file(s) and folder(s) by right clicking on it/them and selecting 'Send To | Compressed (zipper) Folder'.
Also include a dxdiag report for both systems (WinKey-R, type in dxdiag and hit enter, then when it finishes loading click on the 'Save All Information...' button and save the report somewhere handy).