My friend times out and crashes when connecting to my server

Help me please!

Everyone can join my server, but my friend cannot!
When he joins, hes in for less than a second in the game and then the game crashes! (It just closes, doesn’t say anything)
In the Server Console it says:

Client "friend" connected (XXX.XXX.XXX.XXX:PORT).
Dropped friend from server (friend timed out)

Do you know how to fix this?

That error doesn’t really reveal anything, it’s just saying that he lost connection (which obviously happened because his game crashed). It sounds like a problem on his end rather than your server. Your friend should try removing all his addons/workshop subscriptions then try joining