Fix for odd Messages behavior on Mojave?

A relative is running Mojave (10.14.6) on a 2015 MacBook Pro (and can’t upgrade). Messages has started to act oddly — an iMessage sent to one person appears in a conversation with another (but on his phone it’s all correct, and the correct person got the message).

I assume this is a problem with whatever database keeps track of messages. Is there some way to do the equivalent of Rebuild Mailbox from, or does anyone have experience with this?


Have you tried signing out of iMessage then back in? (Maybe throw a reboot in the middle there)

I think they have, but will make sure. Thanks.