I would not worry to much about it. While I do not know this individual, he does sound like a scammer. A good way to look at it is that if he could put you in a coma so easily then he probably wouldnt need your money. there would be many more lucritive ventures open to him. I really cant see asking for money for doing spells. (maybe components but those usally are pretty cheap. Besides If the spell didnt work then why should he be paid.
hope this helped