Trying to correct someones screw up usually takes twice to three times longer, rather than them bringing it to me first. I try to be fair with fellow enthusiasts, but sometimes they really try my patience and I've lost a few due to labor costs.
Sometimes better to tell a potential customer you do not want to do the job or that you are too busy. Better to write off the 1/2 hour to inspect up front, than try repairing a more or less losing situation.