My friend thinks the exchanges also go down when assets "flash-crash", because it means normal users can get in to sell their assets. And when they can't get in, they also can't cancel their stop-loss orders, so it means a TON of assets are available to buy cheaply (by the exchange and/or certain VIP users/collaborators) as the value goes down. Then these are bought up, and the price quickly increases again, before the exchanges slowly come online again.

Therefore he thinks the people exchanges are either part of the "flash-crashes" (and buy up assets themselves, because they know exactly where most of the stop-loss orders are placed), or they get kickbacks from the whales manipulating the market, by giving them access to the database with all the limit and stop-loss orders.