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Disclaimer: None of this constitutes proof; it's just an informed guess. Any corrections with reputable sources are welcome.
The simplest proposal seems to be single-electron transfer (SET) from $\ce{Cp-}$ to $\ce{Fe^3+}$. This forms the neutral cyclopentadienyl radical as well as $\ce{Fe^2+}$. This $\ce{Fe^2+}$ then reacts with any remaining unreduced $\ce{...
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The short answer is yes, the spin state of the system must change, and you are right to question how that could happen, given that spin changes are generally considered "forbidden". I put forbidden in quotation marks, because these reactions in reality happen quite readily despite the prohibition.
As for "how does this happen?", the spin-...
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