Viva voce question about another student "validating" my results

I suspect your supervisor was thinking about how that student confirmed your results. A classic way this could happen is, if you solved a problem with one method and then the other student solved the same problem with a different method, and your results agree, then the other student has "validated" your results. Naturally, you don't need to have worked with each other to do this - in fact it's arguably preferable that you don't work with each other, to ensure independence of results. Neither do you need to be able to cite their work (you can just say "unpublished").

It's always good to have someone validate your results, because it increases the chances that your results are robust and not due to, e.g. statistical noise. If someone has validated your results, it's definitely something to mention.

It seems you are worried that this would undermine your work in some way, but the point is that if someone also obtained your same result independently then two things happen: (1) it is a good sign someone else was working on a similar question, since it shows it is relevant to other researchers in the field, (2) the fact that their result matches your confirms your result is correct. I would add that (3) it is also good etiquette to acknowledge the work of others, specially when they were working in the same field as you. These people may be collaborators in the future, and it is not a really good start if you ignore their research.