This is the best stuff on earth, sold at home depot, you can even drill it when it dries in 20 minutes and can be applied underwater and it will still cure in 20 minutes
[ame=http://www.youtube.com/watch?v=7DszWhHG49c]Glass Blowing. How to fix a broken glass pipe. - YouTube[/ame]
^that is completely not a plausible method to fix his break. Just buy a new bong and don't get cheap china glass.
]TAKE A CONDOM STRETCH IT OVER THE BOTTOM AND WALLA OR A BALLOON IF YOU COULD STRETCH IT THAT BIG TRY IT 
I can only imagine when his friends come over and see him hitting a bong with a condom wrapped around it LMFAO
I'm thinkin the silicone idea would be the best laid plan. But more importantly, don't listen to those who are tellin' u not to try and fix it. It's at least worth a an attempt. Never desert a pipe or bong untill it flatlines. I don't toss anything until I've tried to fix it. Plus, it's bragging if ur successful. I wish u was one of my friends, I'd already fixed it for u.
Better idea, take two inner tube patches and put one covering the hole on the inside of the bong and the out side. I promise it will hold forever and be water and air time. I might look a little funny but look at it this way, ur bong will have mega character, it would be on of a kind, and really kinda epic when u think about it.
Dont use glue of any kind, you will get cancer, just duct tape the piece of glass to it, itll be airtight, and good enough for a broken bong.
I'd rather stick a bong in between my ass cheeks, take a pic of it, and then title the thread "how to fix a bong in a crack" Hahahahaha now that right there is funny i don't care who ya are.
Salvage what you can, bowls etc. Buy yourself a new one, and next time toke somewhere soft away from things that might damage your glass.