Fix for odd Messages behavior on Mojave?

Software
1

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 Mail.app, or does anyone have experience with this?

Thanks!

2

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

3

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