Did you forward the port 51318 both for UDP and TCP in your router? And make sure you have a public IPv4 address otherwise it won't work! Then in the menu your friend needs to enter <YOUR IP>:51318 and press join.
I'm sorry that playing multiplayer is kind of difficult to set up but this game was made during a game jam so my time frame for implementing a proper server system was pretty slim. But thank you for trying out!