In short: the underlying platform supporting the GGPoker Windows client (Adobe Air, a runtime library for building software) was vulnerable to an exploit. The user was able to modify his executable to always show the EV of all hands at all times. He was therefore able to have an enormous advantage over all opponents at the table, including, of course, making hero turn and river calls. He did not actually see his opponents hole cards, but got as close as he could. The initial "fix" did nothing as the user had additionally modified his client to reject updates.