Messages.app on macOS won't send/receive/display messages to/from one specific contact
This post solved the issue. On your iPhone:

Open Settings.
Scroll down to Messages.
Tap on Message Forwarding.
Ensure that all the devices are selected.

I just switched from my old MacBook Pro to a new one, and I didn't even think that I'd need to do something like this.  Still not sure why it affected just this one user, but after doing this, the problem was resolved.