Hopefully the 'mechanic' stopped the instant he felt resistance
The "100k plug life" cars have a very bad habit of doing this.
The plug gets fused to the alum. head over the years.
If it is going to come out, the following is my best procedure;
First you need to clean the base of the plug at the head very well to remove any dirt and $tuff.
Next you need to soak the base of the plug at the head with a VERY good quality penetrant oil - I use GM/AC Delco Part 10-6000 (dealer #88900415)Rust Penetrant and Inhibitor. It will stick well where sprayed and really "gets in there" to release the threads.
Once it's soaked overnight, respray right at base and try loosening the plug and turn out until it stops - respray and turn back in to seat ... repeat until you can back it out all the way.
If it still won't back all the way out it is probably pulling the threads as it moves and there will be no way to get it out and save the threads
If it does ruin the threads coming out then you will need to "heli-coil" the head to repair it.
It;s really not hard and SO common that all parts stores sell a little 'kit' to do this.
Hope this helps and you get er-done