Can a minority shareholder sue if they think a sale unfairly favours the majority owner?

Yes — this is precisely what Ontario's oppression remedy under the Business Corporations Act is designed for. It allows a shareholder, including a minority shareholder, to apply to a court for relief where the corporation's affairs, or the conduct of its directors or majority shareholders, have been carried out in a manner that is oppressive, unfairly prejudicial to, or that unfairly disregards the interests of a shareholder — which can include a sale structured to benefit the majority owner at the minority's expense.

A court has broad discretion in what it can order if oppression is found, from compensation to orders affecting the transaction itself, but success depends on showing the conduct was actually unfair in a legal sense, not just that the minority shareholder would have preferred a different outcome. Because these claims are fact-intensive and time-sensitive, a minority shareholder with real concerns about favouritism in a sale should get legal advice promptly rather than after the transaction has fully closed.