The claimant was a bus driver. He'd been dismissed for giving his high visibility vest to someone else but won his internal appeal. Nonetheless, he lodged a claim that the action originally taken against him was victimisation because he was associated with a third party (a union) that had done a protected act.

The act in question was that some union members had complained about the employer breaching the GB Equality Act and that he had overheard this and relayed it to a manager, who then, the claimant alleged, associated him with the group of union members. The relationship between the claimant and those doing the protected act doesn't actually have to be all that close:

"The question was whether the Claimant’s treatment was by reason of the protected acts done by others. This test would require no particular relationship to be established between the Claimant and the others. It would be a question of fact for the Employment Tribunal whether the employer subjected to the Claimant to treatment by reason of the protected acts of others. The association might be wholly or in part in the mind of the employer... Association is not a concept in itself found in European legislation. The issue is not whether there is in existence a relationship of some particular kind but whether in the mind of the putative discriminator the protected act of a third party was part of the reason for the treatment of the employee."
http://www.bailii.org/uk/cases/UKEAT/2015/0108_15_2007.html