ok. well, that centre bolt is only really for adjusting the headset... the two allen bolts in the stem are there to hold everything together, so it should never really be tightened that much.
Your mate should be able to get the top of the bolt our by simply unscrewing it, release the forks by undoing the clamp allen bolts then attacking the remaining broken bit in the steerer tube by whatever means available, it shouldn't be in any way tight with the top part off...
Does that help?
This is the first one i've heard of snapping, by the way
