Probably from the plum curculio weevil.

https://site.extension.uga.edu/bartow/sappy-peaches/

tj

The article at the above link recommends applying an insecticide at: "shuck split". That's also the time to apply a fungicide (Daconil) to prevent brown spots. As I read the Daconil instructions you do not want to apply it after the fruit has formed. Which scares me off of using it on tomatoes or squash/cuke plants that are fruiting.

Thanks all. I'll let him know and he can do the research!