Is it ethical to have results in your PhD thesis that are not strictly yours?
It may be good to report results that is connected to your reports in your thesis even if you did not directly contribute to it as long as you attribute main contributor of the result from an ethical stand-off. But this would not be applicable if the concerned result is to play a major role in your dissertation. I which case, IMO, you try to do something more or different to make up for the major result.

Apart from my views, the your thesis advisor ought to be the best person to clarify this particular doubt.