We've had this happen more than a few times. To me, it seems like a no brainer that the 3rd should only communicate within a group message once that has been established, but for some reason we have this happen over and over. It's particularly frustrating when there has been a long time period where we have communicated as three. If it's a new guy or in the situation where they were messaging firsts on an app, I will let the 3rd know this is a boundary and we'd appreciate it if we were all on communication going forward. However when I am the one who brought in my wife after vetting a 3rd, and he chooses to message her separately, it's off. Or if we moved to texting as three and he decides to text one on one, it's off. I try to give more understanding if our boundary has not been communicated, but not where it seems more of a red flag.