IMO MC may have needed the liquidity in case any investors requested their money back. Getting the money from her parents may have been timely to pay back an investor in the Ponzi scheme? Maybe there were no reserve investors waiting to join.... We would need to understand the timing of any previous returns to investors and available cash flow at those points in time. Perhaps cash was also needed for overseas trips/ purchases? MCs parents cash could have helped shore up loans (depending on their timing).
Then there is the negative gearing aspect or possible losses to offset future capital gains (if in trust). Was the alleged rent under market value? Do we know what rent was being paid and have the renters names been confirmed?
In any case I feel very sorry for MCs parents.