"Who needs friends when memes are forever"
With these ＡＥＳＴＨＥＴＩＣＳ you didn't stand a chance...
I know the exact issue. Your friend recently bought the game, right? Did he ever play vanilla multiplayer? If not, he does not have a SoSE online account with Ironclad Games. Disable the mod in the options to get to the original game, then click on ICO/Multiplayer. Make sure there is an account linked with his game (he will have to make one); then it should work. SotP uses the Ironclad servers to play matchmaking games on, so without an account he won't be able to play online; and thus get the error he was getting.
I had this same exact issue when I was first playing. If there is no account linked in the vanilla game, it will crash and give you that exact error. Try having your friend create an account within the vanilla game, then re-enable the mod and try again.