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Can someone provide a reference for the following Maschke lemma:

If $C$ is a semisimple algebra over a field of characteristic zero and $G$ is a finite group acting on $C$ by automorphisms, then $C \rtimes G$ is semisimple.

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Theorem 7.5.6(iii) of the book Noncommutative Noetherian Rings by J.C.McConnell and J.C.Robson states the following:

Let $R$ be a ring, $G$ a finite group with $|G|$ a unit in $R$, and $S = R \ast G$, a crossed product of $R$ by $G$. Then the right global dimension of $R$ equals the right global dimension of $S$.

An algebra is semisimple if and only if it has right (or left) global dimension zero, and your skew group ring $C \rtimes G$ is a special case of the more general crossed product construction. So your Maschke lemma is a special case of the result cited above.

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    $\begingroup$ +1. It's good to remember sometimes that Maschke's Lemma and Theorem are really statements about global dimension. This is often the right point of view to generalize them to other categories $\endgroup$
    – David White
    Commented Aug 6, 2013 at 15:01
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This is also stated in the generality asked for as Theorem 1.3 (c) in [Reiten, Riedtmann: Skew group algebras in the representation theory of Artin algebras, Journal of Algebra 92, 1985].

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