Why was the accounting firm's claim against its former client considered valid?

The claim regarding the accounting firm's case against its former client is considered valid primarily because the claims were transactionally related and should have been asserted as a counterclaim. In civil procedure, when a defendant has a claim arising out of the same transaction or occurrence as the plaintiff's claim, it is often required to be raised as a counterclaim. This ensures that all related disputes between the parties are resolved in a single proceeding, promoting judicial efficiency and preventing piecemeal litigation.

In this scenario, the relationship and transactions between the accounting firm and the client are significant. If the issues at hand stem from the same dealings, then the proper procedural mechanism would dictate that they be consolidated as part of the same litigation rather than being treated as separate claims. This interconnectedness gives the accounting firm's claim a solid footing when viewed through the lens of procedural rules relating to counterclaims.

This approach contrasts with other options, which do not address the necessity of transactionally related claims being brought together. While the breach of contract could be a component of the firm’s argument, it does not focus on the procedural aspects that validate the claim without going through the proper channels of asserting these related claims in a counterclaim format.