Messages I did not send showing up along with my own undelivered messages?

I tried texting a photo to a friend, but got the error message "Free Msg: Recipient unable to receive message - Message Blocking is active". But the weirdest thing is that along with the photo & caption I typed, another message that I did not type also showed up as undelivered: "I threatened her with causing you to leave her." I have no idea what this is referring to and I did not type it. I tried sending the photo one more time, and another message I did not type showed up: "for any reason, ever". Am I seeing someone else's messages to him? Because that seems like a huge breach of privacy. I thought maybe it was an issue with my phone or that maybe he had blocked me for some reason, but I was able to text another friend with no issues, and then I saw on Facebook that his phone is turned off for a few days. So that explains the error message but not why someone else's texts are appearing as my own.