Interesting situation.
Buying a house was foolish.
They need to pay the money back.
However, it was.in their account, so who gets the interest is questionable.
Slightly different scenario.
A former co-worker once got a paycheck for 800 hours instead of 80.
He set the extra in a CD and left it alone.
About 5 months later the finance department was freaking out and he told them to just wait a day or two so he could move the money.
He said the interest wasn’t huge, but a good little windfall that he didn’t have to do anything for.