I think I would have been thrilled at their thoughtfulness, and thanked them profusely. Later I would have told them that it didn't quite work, and asked where the old latches were.
Reminds me of a true story a friend told about himself, his sister and his father. Dad bought a new car. it was about 2 months old and the kids decided to surprise him by washing it. The worked on the car for a couple of hours, and were very pleased with their work. They called Dad out to see the wonderful job they did and to get the expected praise.
Dad was pleased at how thoughtful they were and thanked them. He never said a word about the scratches rubbed into the car by the BRILLO pads they used to clean the car.
Now they all laugh about it. I laughed, too, but I don't think I'd have had the same reaction as he did!