In C-1/17 Petronas Lubricants, the CJEU held end of June, entirely justifiably, that assigned counterclaims may be brought by the employer in the forum chosen by the employee under (now) Article 20 ff Brussels I Recast to bring his claim. In the case at issue, the employer had only obtained the claim by assignment, after the employee had initiated proceedings.
The Court pointed to the rationale underlying Article 22(1), which mirrors all other counterclaim anchor provisions in the Regulation: the sound administration of justice. That the counterclaim is merely assigned, is irrelevant: at 28: ‘…provided that the choice by the employee of the court having jurisdiction to examine his application is respected, the objective of favouring that employee is achieved and there is no reason to limit the possibility of examining that claim together with a counter-claim within the meaning of Article 20(2)’ (Brussels I, GAVC).
Evidently the counterclaim does have to meet the criteria recently re-emphasised in Kostanjevec.
(Handbook of) EU private international law, 2nd ed. 2016, Chapter 2, Heading 18.104.22.168.