If in America, speak to the DOL and/or your legal counsel. If you deny interviewing a qualified candidate due to discrimination based on their affiliation with another previous employee, that may be construed as a personal act of discrimination by the company, and they could potentially file suite.
Since you are actually considering not doing so for that exact reason, you had better address this immediately to make sure you aren't in the wrong, if this candidate was practically a shoe-in for the position, which should to be the case, then, they may actually have a reason to pursue your organization, even if there is no valid case, there may be enough to bring to court, in order to try to force a settlement.
My wife (a social worker) explained that you MUST NOT speak to him about his wife in any way, as this violates employee confidentiality, and that HR should be keenly aware of this and present on the call to protect you from speaking about that situation as its required to be completely confidential to the employee.
Furthermore she indicated that if the interviewee brings it up you need to say she worked there but you aren't't able to discuss the details about former employees and hand it over to HR if pressed, as its their job to protect you from being exposed.
Also she said it if there was any pattern of intentional sabotage after the employee were brought on board that would be grounds for firing any employee and possible legal action against them.
Finally she agreed that if this reason is not a valid one to use to evaluate whether they are fit for the position.
-now, I'm not a lawyer, but you seem to have already placed tourselves in a disadvantaged position by considering his relationship to a previous employee and making it a consideration, if a lawsuit alleged that they had reason to believe this was a factor in their not getting an interview they could file suite and request any correspondence employees made about this, in which case you have the most to lose as the company would likely disavow you being an agent of theirs in making these inquiries and providing the advice to colleagues.
So, again, speak with your HR, let them know your concerns and they you reached out to that hiring manager, and as not all HR are trained social workers and up to date on all DOL guidlines, you should get your company social worker and Lawyer to assist them in making sure you properly handle tbs legal obligations to all parties.
Your most important thing to do is distance yourself from affecting the process negatively or you could be personally liable.
most importantly, as said by others, consult a PERSONAL Lawyer specializing in labor before doing anything, you may want to discuss doing something along the following lines with them:
It may be all you can do, is damage control now. Ie. shut your mouth to the manager, speak to HR hiring person and social worker and ask to bring in the staff legal counsel, and hope that they weren't planning to not interview the person based on his association with the former employee.
You can take notes, and if they indicated they were planning such a thing, you could make sure to note that down, then let them know they may be opening themselves to legal trouble if they do so, and let them sort it out.
This way, if they do try to make a decision based on that relationship, and try to blame it on you, you can show that you spoke with them on such and such date and they were already looking to make that desision, and you informed them that was not the proper course and advised them to look into it if they try to make it your personal legal matter. The company would be trying to get out of suite by pushing liability to you if they can, but the plaintifs would rather get the deep pockets of the company than your personal bank account.
Likewise if the company tries to fire you as part of that you may havs grounds for wrongful termination at that point.
And most importantly, as said by others, consult a PERSONAL Lawyer specializing in labor before doing anything!
I want to be clear I am saying you should only be concerned about a civil law suite because you have indicated that you interjected into the process, offering viewpoints, (even-though pro-interview viewpoints), and indicated that there isn't a clear guidance coming from the HR, in fact HR should not have informed the hiring manager of the relationship the candidate had with the previous employee, but should have stated that they needed to be present for all interviews with that candidate only, as by informing the hiring manager they are breaking the employee confidentiality. So actually it sounds like they may have opened grounds for a civil suit against them if I understand you correctly, and in either case, your involvement opes you up to grounds for a civil lawsuit.
I'm not even saying it would be a GOOD lawsuit, but the standards for Civil cases are much relaxed from criminal cases, and you should do what you can to make sure that if such a suite were to come forward you couldn;t be fired with cause or dragged into or perhaps even scapegoated onto.
As regardless of the motivations of the candidate, if they are string along and get the impression that they are being discriminated against due to their wife they may sue, and for a civil case you can get a lot further based on what you believe happened than with a criminal case.
and so, again, I must say, contact a personal lawyer and discuss this matter with them and ask about if you should bring them in to speak to your HR's hiring Person (and HR Social Worker if not the same person), or maybe avoid that conversation entirely.