Try to restart your router.
It's not the router if other servers are working. The IP is correct, try to re-add the server to your favourites!
Close SAMP > download new version > install. If you still have 0.3e open when you try to install, it won't work.
If he uses 3e and Argonath is on 3x, it will still show the server name, player amount, ping etc. it only says "Server didn't respond" when you try to join it.