From a mechanic's perspective, the answer can be difficult..
This mechanic should come back to fix this issue, without question. However, the quality of the mechanic is now in question. If you chose this person because he was cheap, you just found out why he's cheap. If he touts himself as being a high-level mechanic, then he should stand behind his work. Everyone knows that mistakes happen. Sometimes, it sucks having to correct them, but any reputable mechanic would make this right.
In some cases though, you may have to accept it as an expensive lesson, and find someone else to do the job.