This sale has two situations:
1- If the seller made a mistake and gave her the genuine item at the imitation price, and he was deluded in doing so, in this case, the sister must return to the seller and inform him of what happened after confirming the real price from multiple sources. In this situation, she has the right to either get a refund for what she paid or pay the price difference.
2- If the seller did not make a mistake and was aware of the value but, for some reason, gave her the genuine item at the imitation price, in this case, it is considered a gift, and she is not obligated to do anything.
And Allah knows best.
Fatwa by Dr. Khālid Naṣr