This. Your customers are not idiots, they're non-experts who chose to try to repair something on their own, possibly learning something in the process. Good for them! Sometimes they might succeed, sometimes they get stuck and bring it in to a professional. You are entitled to tell them, in a non-accusatory tone, that due to previous repair work that was attempted the job will take longer and you charge by the hour.

That's the risk they accepted when trying to DIY and there's nothing wrong with that.