My understanding is companies rarely resort to suing former employees to recover bonus payouts, as it's a lot of work and generally deemed not worth the time and effort to go through the full drawn out legal process. The legal advice I received about this as a hypothetical situation / concern was to simply not respond to any further inquiries from the company after resigning.

This particular case is egregious since she only ended up with 10% of the bonus amount as cash due to ridiculously high moving costs.