It is impossible for God to heal someone at the behest of one of His saints in heaven.
I suspect the answer you get will be doused in the supralapsarian predestination pap that Cauvin sprayed around like a lawn sprinkler.
Your question itself is faulty and presumes something not in evidence.
There is no Scriptural support for the extra-Biblical teaching of Rome that says saints in heaven intercede on men's behalf.
We do, however, have Scriptural evidence which disputes that error since the Bible tells us the only mediator between God and men is Christ Jesus.
And starting a sentence with "Is it impossible for God to (fill in the blank)" is a child's game. I don't play it. I stick to the Scriptures for my answers.