Miata NB cam angle sensor - bolt broke

I wasn't home this weekend and my son wanted to change the CAS on his 2001 Miata.  Simple, right?  One connector and one 10mm bolt right on top of the engine, what could go wrong?  Bolt head twisted off.  We've got a couple of threads exposed over the deck to work with.  Kroiled overnight, grabbed on an won't turn.  

I am thinking about talking him through dremeling a slot into the top to try to turn it out.  Other ideas?  Or what's the likelihood it will be fine with no bolt for a week?  Will the pressure of the new o-ring hold it in place well enough?  At worst if it doesn't start he could just shove it down to reseat it?

Trying to decide which is worse - no bolt or talking someone inexperienced through this recovery.

Nut on it and a weld between them. That should get it out. If no welder then you can JB weld the nut on but the heat of the welder helps sometimes. 

What ever you do, cut, weld, etc make sure you keep the sensor hole sealed up so no grit goes down the hole. I vote weld a nut on. the heat is most important.