maybe he has a hacked\cracked\"forbidden to talk about stuff"(WAREZ stuff)stuffs
so his serial wont do it!!!
Or another guess is that that server is over flooded or just too full or something like that
if you are playing in ur friend house so dont wait for help from any mods cause It`ll take em some time to help you and unless ur there for a long time , I guess that u`ll just have to wait