If (and only if) one party has actual evidence or data of rigging, it wouldn't be hypocritical for them to challenge the results after not taking Donald's whining during the campaign seriously.
But it's academic, the results shouldn't be overturned. Even if it was found legitimate to give Hillary the win, it would make a martyr out of Donald and his movement would gain traction for 4 more years. The next 4 years are going to be ugly either because Donald is president or because half the country believes a conspiracy gave it to Hillary.
One party or the other is going to be punished in 4 years. Better it should be Donald, let him fail and have his movement get put behind us for good.