Judging from the report, it seems there's a conflicting port on his machine. It
could be a super obscure bug where the program starts up the connection multiple times as well, but let's see if this works.
These instructions will seem very weird, but uh... just follow them
If your friend updates the game now, goes into Options and then exits options, then exits the game, there should be a config.txt file in %APPDATA%/Secrets of Grindea.
Tell him to open config.txt and edit the value under Default Port to something else (say 15521 for example), then start the game and attempt to host a game.
If the game no longer crashes for him, you can connect to him by entering [IP]:[port]. Example: 81.239.4.52:15521
Note that he'll have to open up this new port for the connection to work, and just ignore everything about port 6000 (the default port)!