I put the note in our neighbor’s mailbox on Tuesday, and spoke with the mom yesterday. Her son never mentioned the incident. She said that boys will sometimes conceal information like this, unlike girls, who might have more trouble keeping it in their brains. She was happy to know about the window, so that she could talk about it with him.
She wanted to pay for the cost of the repair, and take full responsibility. I told her that her son wouldn’t have been in our yard if my daughter hadn’t thrown a water balloon over the fence. And it was great that they were finally spending time together, as neighbors. So, we decided to split the cost. I told her the repair guy did a terrific job and that I’d save his phone number in case we needed it again.
Pamela Gwyn Kripke 
Sounds like a fabulous resolution with good feelings ruling. Lessons can be messy but there’s no reason they have to hurt. IMHO anyway.