Why were you taking that bolt out?
Does it matter why? It's snapped and needs a answer.
I use bolt extractors. You need to drill a hole the the bolt. Then then turn the extractor in the hole you drilled. The thread is opposite, so as you turn and tighten the extractor anticlockwise the bolt should undo.
They look like this

Photo0374 by tweedub, on Flickr
Would like to see you drill through high tensile steel on a crank tbh... Especially when you think thats a pritty f**king tight and large bolt too. And obvioulsy snapped with a socket on it and probably a huge bar. So an easy out and vice grips wouldnt cut in my opinion....
And reason i asked was because i had a feeling he was maybe thinking that crank bolt was for taking the bottom aux belt pulley off to get the bottom timing cover off. Iv seen people think that when they take out the allen key bolts but the aux pulley still wont come off because its seized to the crank pulley. So thats why Tweed i asked my question... So i grand sceme of things... Could actually matter.
He's drilling a bolt not a high tensile steel crank numb nuts!
The head of bolt would have snapped off, so it won't be tight any more. It's just a thread in a hole.
What do you think drills are made of, chewing gum?
Use metal drills with cutting fluid to keep it cool.
Your question still means nothing! Even if he didn't need to undo it, he still has snapped it and needs it removed.
So if my way is wrong and you doubt it will solve it, maybe in your next post tell us how to get it out!