Every time someone ask this question I think, It's not what you should do. It's what will the next guy do?
You are the nice guy. You take the time, Look it up. Find a reason to pass it. The next guy is just fucking LAZY FAIL. DING.
The reason the guy didn't come back to you was you took to long to get his car done. So now he looks for a new guy. You helped the guy out and he goes somewhere else.