Please need help!! Minecraft download error!!?

my younger brother downloaded the new update yesterday and it did not work when he tried to play multiplayer - so we found how to get back the old update 1.6 i think . Once we did that it work fine . But today it does not work. When he tries to go on to multiplayer it said CLIENT OUTDATED. so we googled it and thought we should download the new update. Big mistake. For some reason my brother is on the servers and is playing his. i think the server is global domination hq . anyway my brother found a mod on his thingy and clicked it. and know when ever he goes on to minecraft it is just a black screen. Please help!!!!!