I've always found this a baffling way to word a question.
What's the difference between these two?
Is it true that your relationship with my client is entirely platonic?
Isn't it true that your relationship with my client is entirely platonic?
Is the second one asking if it is true or if it's not true? Given context, they seem to be asking the same question with the same meaning. But the second one is just so damn awkward to me.
I had a coworker that would always ask a question as a follow up to a comment in the negative:
You haven't seen John, have you?
You don't have a pen I can use, do you?
You're not parked out front, are you?
Is it true that your relationship with my client is entirely platonic?
Isn't it true that your relationship with my client is entirely platonic?"
"Isn't it true" has a negative connotation. "Is it true" is a simple question.
You see lawyers on TV shows say "Isn't it true...?" when cross examining a witness they are trying to discredit. There is a challenge, a negative connotation, that is not in "Is it true...?"
I had this discussion with my wife who watches a lot of true crime shows and listens to true crime podcasts. She said much the same thing.
The context of that discussion was a case where the attorney kept making a statement, then asking, "correct?"
Again, it was very awkward to me.
Instead of asking
Were you present in the office Monday morning?
He would say
You were present in the office Monday morning, correct?
Apparently this is a thing for the purpose of preventing your witness from saying something unexpected if given the opportunity to actually answer, but it drove me fucking crazy listening to the questioning/testimony. It was every single question. Instead of asking a question and having the witness answer, it was a statement followed by "correct?"
>Instead of: What is your name?
>Your name is John, correct?
>Instead of: How old are you?
> You're 36 years old, correct?
>Instead of: Where were you born?
>You were born in Texas, correct?
Yes, of course. I only mentioned this as context to the discussion I had with my wife.
In the context of that discussion, we also talked about "is it true" vs "isn't it true" and why someone would use one over the other. She said much the same as you are saying here.
Basically what everyone else has said, but I think it also kind of sets things up for something else.
Simply saying, "Is it true that your relationship was entirely platonic?" is one thing. But, as someone else said, "Isn't it true" adds a bit of a challenge. For example, it could go something like this:
"Isn't it true that your relationship was entirely platonic?"
"Yes."
"Then why would this witness say they saw you two kissing in the breakroom?"
As far as your former coworker's way of asking questions, that might just be a linguistic idiosyncrasy. It's interesting, though.