Say $G$ is a finite group with presentation $\langle S | R \rangle$ and let $C$ be the commutator subgroup of $G$. Then $\langle S | R \cup \{ sts^{-1}t^{-1} \} \rangle$ is a presentation of $G/C$.
I have been at this for a bit now and any help will be greatly appreciated. We have a map from $f:S \to G$ and the natural map $u: G \to G/C$. It is clear to me that there is a map from $f':S \to G/C$ such that $f' = u \circ f$. Now, I have been trying to find some way of putting $C$ in $F_S$. An idea that I have is to use the fact that $G/C$ is Abelian to infer that all elements of $S$ commute. Then use this information to show $F_S$ is Abelian.
But I am not sure if my approach is correct.
Here is a sketch proof. There is no need to assume that $G$ is finite. Let $$Q = \langle S \mid R \cup \{ sts^{-1}t^{-1} : s,t, \in S \} \rangle.$$ Since the image in $G/C$ of each $sts^{-1}t^{-1}$ with $s,t \in S$ is trivial, there is an epimorphism $\tau:Q \to G/C$.
But since the generators of $Q$ commute with each other, $Q$ is abelian, and so, since $G/C$ is the maximal abelian quotient of $G$, $\tau$ must be an isomorphism.