Broken Blade Bolt on a Toro Recycler 22" Mower - Model #20371 Serial #316125226

My niece contacted me today to say the Blade Bolt on their mower broke off - the other broken half of the bolt is inside the engine shaft.  Unfortunately, I'm 4 hours away and was wondering how difficult of a repair this would be.  Her husband is a teacher and she's a stay at home Mom with 2 young kids.
After doing a quick online search, I thought the broken bolt was inside the Blade Driver and figured this might be an easy fix for them, but then realized that the Blade Driver is sliding over, what I think is the engine shaft.  A neighbor tried to drill out the broken bolt, but only ended up breaking several drill bits.  Even if he had been successful removing the broken bolt, he was concerned that the threads were damaged on that shaft.  Before I tell them that this might be a lost cause, I wanted to reach out to you folks to make sure I was diagnosing this issue properly.
Any thoughts on whether this could be repaired (within their capabilities) or by a repair shop that won't end up costing them more than a brand new mower?
Thanks for any suggestions. 
  

SRTsFZ6
With the head of the bolt broke off, there isn't any tension on the bolt any more..
 
IME, the repair would be to weld (MIG) a nut onto the bolt.
Plug weld right down the middle being careful not to weld the crank itself.
 
Between the heat and now something to grab, the bolt come out fairly easily.
 
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Thanks for your reply.  They elected to get a replacement since someone familiar with lawn equipment already mentioned to them that the threads on the shaft looked messed up.