Hi and
to NGA
I think its too late to save to your plant. It should not have been removed from the mother plant until it had its own roots. The only way to tell that is to take the mother plant out of the pot and check the babies.
At this point, you are essentially rooting a cutting - the chances of success are slim.
Next time your friend offers you an offset, make sure it has its own roots, plant in cactus soil, not indoor potting soil and water sparingly. The new leaf you saw was grown from energy stored in the leaves but without roots, it could not sustain itself. The remaining leaves you have are rotting.
Sorry to be the bearer of bad news...
