Things have actually gotten worse since the member started using "reply all" instead of "reply." So I am now strongly urgng Bruce's proposed fix.
What's happening now is that the member thoughtfully clicks "reply all," which makes the message go to the individual with cc to the group. Then when the individual replies, THAT message goes out privately rather than to the group. Meanwhile I've asked the member to carefully check her "to" field and remove the individual member's address, leaving just the group address.
As Bruce says, this is certainly happening more often than we realize. How many people use PCs running Windows 10? That's the most up to date version, right? So probably tons of people.
Messages are the sole opinion of the author, especially the fishy ones.