1. Citing sources that contain citations
In your first scenario, you do not cite Girard. You are citing Cohen who just happens to use Girard to make his argument. Your text is about Cohen, so cite Cohen:
Cohen argues that monsters represent social prejudices: "Monsters are never created ex nihilo, but through a process of fragmentation and recombination in which elements are extracted 'from various forms' (including – indeed, especially – marginalized social groups) and then assembled as the monster, 'which can then claim an independent identity.' (Girard 123)" (Cohen 456)
Please note that I included the source as it was given in Cohen's text (
(Girard 123)). I made this up, as I don't know how Cohen acknowledged Girard, but you need to present Cohen's passage exactly as it was printed, including footnote numbers or parenthetical citations.
If Cohen does not acknowledge Girard in the passage you cite, then I would either mention him in text (see example below), in a footnote, or in a brief parenthetical comment, e.g.:
"..." (Cohen 456, citing Girard 123)
I also made up the part about Cohen and social precudices to illustrate that what you cite is Cohen's interpretation of Girard, not Girard. So even if your text is about Girard, your citation is about Cohen!
Here is an example for this kind of citation from page 175 of Transitions and Dissolving Boundaries in the Fantastic:
List Girard in your bibliography.
2. Citing indirect sources
In your second scenario, you cite Girard, whose original text is unavailable to you and who you can only read indiretly, through Cohen. Your text is about Girard, so cite Girard as quoted in Cohen:
"..." (Girard 123, qtd. in Cohen 456)
If Cohen does not specify the page number from Girard, leave it out.
Again, both the direct and indirect sources must be listed in your bibliography.