I think the plan is for Trump, once he wins the nomination, to ask for a declarative judgment or Congressional resolution settling Cruz’s birther issue...and then put Cruz on the ticket as Veep.
The only person with standing is an opponent in the election contest.
It would also be presidential of Trump to help clear up the Cruz-birther issue himself. To do his very best to vindicate Cruz’ eligibility would make him greater than ever. He could wait until after Iowa, but I hope he does it before New Hampshire.