Tell them you thoughts - as mentioned,
"I think your other child is a prodigy, and this child
is a normal child, who would benefit from a more gradual (step by step) approach. I have tried introducing more advanced technique to them, but they cannot do it."
Another thing you might try is abridging a famous tune, or writing something that sounds difficult but is easy to play.
That kid is going to have a bumpy ride (in all kinds of ways) if this need from the parent to compare children persists.
And if they insist, if it were me, I wouldn't assist in this path of destruction, and tell them:
'I can no longer teach the child'.