Getting roped into a permie's disciplinary

[WordIsBond]

I actually wouldn't say or do anything until approached. Just don't get involved and pull the "Sorry, but it really wouldn't be appropriate for me to comment on an internal HR matter and could be legally detrimental for both of us if I were to do so" line

This. As long as it is an internal HR matter, that is.

With a caveat already mentioned -- if you are a witness to illegal or dangerous behaviour, you talk.

But if it comes down to endangering your contract or the guy who signs the timesheet makes a personal request for you to talk to them, then he's the guy you need to keep happy, if you can.

Re: IR35, it should not impact your status one way or another to take five minutes telling someone what you've seen. There's no reason an outside supplier who is frequently on site can't do that. But if they ask you to go to a disciplinary hearing or anything like that, get it written up as a separate purchase order or something. That's not what you've signed up for.