Weird additional text on post typing form!?

[NIBS]

So, a user on my forum gets stuff like this on the post typing form when he tries to reply to a thread:

https://ibb.co/mzzncbH
https://ibb.co/5rg8n5w
https://ibb.co/CV9QG3Y

Of course, the form should be completely blank. Furthermore, apparently he is not able to remove that text from the post, as even if he does it still appears in it after clicking Send. He is using Google Chrome on a laptop. He is posting on a normal Registered user account with default permissions and no adapted settings whatsoever. The mybb version of the forum in question is 1.8.18.

I am not able to replicate that problem on rmy desktop nor on my tablet. I don't have a laptop so I can't test that. I tried with both Google Chrome and Firefox, and on both my admin account and on a normally created registered user account. Everything works just fine for me.

Also, no one else on any of my forums has reported anything like that, so I suspect its something on his browser's end related to something on mybb but can anyone please throw any ideas as for what might be going on? I have never heard of this kind of problem before so I don't even know where to starts trying to figure it out!

The forum is private thus I can't post the link. So if anyone could throw any ideas based on what I've told?

[Crazycat]

Your user probably copy&paste from another site (a translator ?) and some hidden code is copied. As MyBB protects content, the script is showed rather than interpreted.

[nixer55]

Turn on View Source before pasting content.

[NIBS]

Oh, thank you for the suggestions but he's not copying from a translator or any other site. I will ask him of course, but here's why I feel I can safely claimn that he isn't.
Details I forgot to mention:

When he first reported this problem he was clearly intending to type on the forum itself but couldn't because of that problem. He said "I have an issue finding out where to type" because of all that weird text and then he said he will type the draft somewhere else for now. Also, he's a native english speaker and his posts are on the level of a very talented book author and flowing so seemlessly there's no way they've been run through an online translaror at any point. (Side note; the narrative story bit in one of those screenshots is not his writing, it's mine.)

Leading me to that he did mention that all that stuff is below my post, so it seemed he was originally replying through the "Reply" button with the quotation mark right below the post he wanted to reply to. In which that kind of additional text shouldn't be either and isn't when I tried it as in I can't replicate the problem that way either. I did then instruct him to use the "New Reply" button instead---but he's still having that problem.

So, to my understanding that problem occurs for him when he clicks on either of the Reply buttons on the forum, before he adds anything to the form himself. And even when he deletes the offending text and then posts his own content, the additional text appears anyway.

[Crazycat]

Peharps he use a chrome extension which is bugged

[NIBS]

What type of extension could cause a problem like that? Like, which kind should I tell him to test if he has any?

[Crazycat]

I don't know, I don't use Chrome and never add extensions.
Tell him to disable all extensions and enable them one by one, with a test each time.