I don't think anyone is saying that the information could 'bankrupt' an airline per se. However, as with all companies, as an employee you often have access to a lot of privileged or commercially sensitive information.
The system used at many airlines internally that allows employees to list for space available leisure travel shows a lot of company confidential information, and it is bound by strict rules of conduct and confidentiality which it would appear have been breached in this instance.
At United, employees have the opportunity to check loads for any flight systemwide on any given date. The system will show aircraft type, aircraft configuration by cabin, number of revenue passengers in each cabin, and number of 'non-revs' listed in each class of service. Therefore, one can find out extremely accurately how many pax UA is booking in each class on each route on each day, and pick out trends etc. A similar system at AC would offer the same information, all of which is commercially sensitive.
It remains to be seen what the 'real' facts actually are, but if AC's claims are correct then this should be seen as a seriously bad managment decision by some of WestJets top personnel, and unethical and bad corporate practice by WestJet.
And yes, I would agree with some of the views already expressed here. If it would have been WJ making the complaint against AC doing that to them, there would have been an outpouring of sympathy to WestJet, 'the underdog' smiley touchy feely airline. But the fact that WestJet is the alleged villain here does nothing more than to make most people smile and say "WestJet - those cheeky guys, what will they think of next!" and chuckle away.