Answer A.) Speak to your customer and tell them what happened. If it’s a concern for them, you may have to replace the entire window for them.
Answer B.) There’s not much you can do now. However, in the future you might want to let your customers know that you can’t guarantee their defrosters won’t be damaged. In the end, there’s a lot of cheap glass and cheap defrosters being used, and it’s something that’s out of your control for the most part.