If the shop went and used an easy out, I'd doubt their ability to fix the situation.
I would find a machine shop and have them remove the bolt. The shop that caused the problem should pay for it, or get you a replacement head.

You have few options now. The head should be removed, and the bolt drilled out with a drill-press and cobalt steel bits. If they damage the threads, especially if they drill off-center, you're going to run into problems. If they can completely remove the stud, they might suggest a thread insert. In that case, she shop will need to check how thick an insert they can install. If there is too much material removed, you may need a new head.

There are a few shops that can do EDM. They essentially melt the steel by blasting it with electricity. The liquified steel of the bolt will splatter out and you can essentially drill with the electricity. The liquified steel won't bond permanently to the aluminum head, so it can be just brushed off. It also won't stick too much to the threads and can be cleaned out with a thread chaser.

Good Luck and kick some butt when you get to the shop. Or, let them be embarrassed by just acting dumbfounded that they would do something that unprofessional.

Happy Bricking... hopefully soon!
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1990 740 Turbo, on its way to stock specs, maybe beyond