 Solution

Charlie measures in the $X$ basis and sends one bit to Alice or Bob; the remaining pair becomes $|{\Phi }^{+}⟩$ or $|{\Phi }^{-}⟩$, corrected by a local $Z$. Alice then performs standard teleportation: CNOT from $|\xi ⟩$ to her GHZ qubit, Hadamard, two computational measurements, and sends the two outcomes to Bob, who applies the corresponding Pauli correction.