See the screenshot below. A friend asked me about this. Person #1 had a group text thread with Person #2 and Person #3. At the top, Person #2 sent a photo and a message. Person #3 replied. Then there appears a rude reply from Person #1 just a minute later. That reply was not sent by Person #1. Person #1 does not see that message that supposedly came from her. This screenshot is from Person #2's phone.
You can see that someone (Person #2) made an in-line reply to that message, and it was given a thumbs-down. The outgoing messages are blue also. So I believe this shows for sure all messages here are iMessages.
iMessage is a secure system with end-to-end encryption. So my thoughts are that the only way this could happen were if Person #1's Apple login were breached by someone else. I know SMS/MMS spoofs happen frequently and that system is an insecure dinosaur. But do spoofs happen in the iMessage system?
Another possibility is someone close used her phone for a minute. The message does not show on her phone. However it could have been deleted after it was sent. I know you can delete individual messages like that.
Any other possibilities?