Idiom for judging a large topic by a small detail

Your friend was extrapolating:

to project, extend, or expand (known data or experience) into an area not known or experienced so as to arrive at a usually conjectural knowledge of the unknown area - extrapolates present trends to construct an image of the future ( -- Merriam-Webster)

You can now argue that it was an unreasonable or unreliable extrapolation.

To see consumption as such a 'structuring system', which is precisely what Baudrillard's conception involves, does not then, rely on some kind of unreasonable extrapolation from cases such as 'saying it with flowers'. (-- from Consumer Society and the Post-Modern City by David B Clarke, p.59)

"Make a hasty conclusion" can match the example sentence. Hasty has the following meaning in Merriam-Webster:

fast and typically superficial

made a hasty examination of the wound

Superficial is the right adjective to describe:

concerned only with what is obvious or apparent : not thorough or complete

affecting only the outer part or surface of something : not deep or serious

Jump/leap to conclusion can be also considered.

You might refer to her as a blind man describing an elephant.

In various versions of the tale, a group of blind men (or men in the dark) touch an elephant to learn what it is like. Each one feels a different part, but only one part, such as the side or the tusk. They then compare notes and learn that they are in complete disagreement.

Your friend has seen part of the art and has made conclusions as if she has seen all of it in the same way that the blind men made conclusions about the elephant after observing only a part of it.