Nothing to do with a *cape* actually! 

Let me try the short and terse answer.

When the first pronoun in the sentence is epicene (no gender specific), the relative pronoun can be **plural.** 

> *If **a client** (no gender specific) does not like your website, **they** (relative pronoun) will never come back.*


> You can't cast aspersions on **someone** just because **they** are wearing a cape.

Worth mentioning here - [singular *they.*][1]