This situation (interviewing for someone else) is not uncommon in the IT/Software contracting field. Even more common is someone sitting across the room from the interviewee and giving answers. Sometimes through headphones hidden under hair. And I am taking about pre covid times where even if the interview was remote the job was more likely than not in office.

What does the manager do when this happens? I have been there. The answer is simple, I did exactly what I would do if the exact candidate I interviewed showed up but fell short of expectations (say used google extensively, or the questions were theoretical and they were good talkers - happens with QA and Project Manager jobs). If they are borderline, give them a task, see if they are able to complete it. A Manager should do it anyway, interviews are hard. Most companies have probation periods for this reason.

Why does it happen/what’s in it for the candidate/contracting firm? For the candidate a few weeks or months of experience that they can add to their resume. Do it in 3 or 4 places and suddenly you are a mid level developer. Hopefully they also learn a bit of programming in the meantime. For the contracting firm any money is money. Sketchy IT contracting forms routinely do this and worse.