I suppose if a hacker really exists then IHG will be starting a lawsuit. Afterall I'm sure they are insured against hackers. Of course it would look bad for them to start a lawsuit and have Police come in and find out that it was in fact a fat finger of a disgruntled employee.
My employers have placed a lot of cyber-liability insurance over the last decade. IHG would be the type of firm that generally would buy this kind of coverage.
A disguntled ex-employee would likely be covered by the carrier. Of course IHG would actually have to file a claim and assist in its investigation.
What isn't going to be clear is what the actual damages are. An insurance carrier might argue contractually that the true incremental cost of the almost free rooms are a fraction of the rack rate.