Salem

while in a party with me. He's now been standing in the river for over 24 hours and I guess he's not able to login. (I only met him in the game a few days ago so I have no other means of contacting him.

I know this could happen sometimes in HnH as well, as long as you were in the party you would still be considered logged in, but I ended the party as soon as I realized.

Sometimes the application doesn't fully close if it crashes, tell him to make sure java isn't still running in the task manager and if it is to end the process.
(when you do find a means of contacting him that is, or if he reads this thread)
There's also times when it doesn't log out properly and keeps your character in game.

It could be that he's standing in the middle of the river, too. However, he should be able to log back in as long as the server has him as "disconnected".