I agree with OliviaOf course not! First off, it would be none of my business, and it would take a mean, resentful person to bring up such a thing and possibly cause problems between my elderly parents or least make them (even for just the one of them) feel bad about something that happened in the distant past. OMG
A couple of words in this original post jump out at me - 'positive proof'. I have this vision of some old private detective's video-tape evidence. Does anyone have a VCR? Anything less would hardly be positive proof. Even discovering you are not the biological child of one parent doesn't necessarily mean an affair.Let's say your parents were in their 80s and you found positive proof one of them had a brief affair way back when they were young. Would you do or say anything to them or blow it off as a nothing burger?