It’s pretty simple, the candidate didn’t meet the requirements.

Not sure why you’re involved, this is an HR and legal issue. If HR said they don’t have docs showing eligibility, then that needs to be reported to the recruiter, otherwise they’re going to recommend this person to somewhere else, and run into the same issue. How would you feel if you were in their shoes?

Whether someone can legally work somewhere is not your pro lem to solve, unless you can actually do something, like help them submit appropriate paperwork (I’ve done this for employees who were temps, to help them get more permanent status).