You can actually insist that ATOS record the interview. I've done this. They give you one copy, and they get one copy.
You can state that you want it recorded in the "any other info" box at the end of the form - I know you've sent that back already, but don't panic. They completely ignored my recording request on the form, and I had to write and remind them when they actually organised the date of the interview.
At the very least, your relative's support worker should take notes. And if, as has happened to me, the interviewer tries to tell them not to take notes, ignore and carry on - noting the fact that the interviewer is trying to stop them!
Also, be mentally (and practically) prepared for the fact that the interview may be cancelled on the day. I don't know what your area is like, but since the new assessments were brought in, for every assessment I've had, the interview has been cancelled at least once. And even on the days I actually was interviewed, they were phoning to cancel people by mid morning, and sending people away who had already arrived - some for the second time.
I gather this is because they book at 130% and rely on sending people away, rather than risk being at less than 100% utilisation.
Anyway, as I say, your area may be different, but your relative may find it quite disturbing to psych up for the journey and interview and then have the rug pulled at the last minute.
Yes yes to what PP says about getting the report afterwards. Some of mine have included comments which clearly pertained to another interviewee (eg "Texted on mobile phone after taking out tablets" when I don't have any tablets and my phone had never been out of my handbag). Hopefully your relative won't need to make a complaint about the report, but it's good to have it there if they need it.
Good luck with it all.