Learn English – “Pandora’s Box” metaphors

The Wikipedia article on Metaphor agrees with your colleague:

Metaphor also denotes rhetorical figures of speech that achieve their effects via association, comparison or resemblance (e.g., antithesis, hyperbole, metonymy and simile, which are all types of metaphor).

(emphasis added)

The article cites "The Oxford Companion to the English Language (1992) pp.653–55" as a reference for the above information, but I don't have access to that publication to verify the statement directly.

Edit: Well, it turns out that I do have access to that reference. Here it is, from p. 653, paragraph 2:

METAPHOR ... (1) All figures of speech that achieve their effect through association, comparison, and resemblance. Figures like antithesis, hyperbole, metonymy, simile are all species of metaphor.

A Wolf in sheep's clothing metaphor may also fit the context.

is an idiom of Biblical origin. It is used of those playing a role contrary to their real character, with whom contact is dangerous. As a fable it has been falsely credited to Aesop and the theme is now numbered 451 in the Perry Index. The confusion has arisen from the similarity of the theme with fables of Aesop concerning wolves that are mistakenly trusted by shepherds; the moral drawn from these is that one's basic nature eventually betrays itself.

Source:Wikipedia