Post up a picture or two, it might help to see what all is going on. Get some close-ups if you can.
Regardless of what takes place after you work on it, you should have them sign a waiver prior to starting, stating you are trying to refurbish this sign, without knowing all the pertinent information and cannot be held responsible for future failures.
You should be able to tell just looking at it if it was a 1/2 a$$ed job or something just went wrong. We've already fixed other people's work, but not without verbally and written dialog, letting them know we aren't responsible if anything else goes wrong.... even if the same area acts up again.
Not many people would purposely sabotage a sign to fail.... it doesn't look good in their resume, so it's probably just a freak accident, but without knowing first hand, I'd be hesitant to do this.