One reason people invest in stocks at the moment is hedging against a falling dollar. In a great market turn, it could be that those $500,000 are the price for a cup of coffee. If you lose access to your shares at that moment, it doesn't help if you are rewarded with money.

*edit: Apart from that: is that actually an event covered by SIPC? Essentially it is a bad investment decision. You agreed to lend out those shares so you have to bear the consequences.