I watched a show in which person A tells person B they want them to come back. After it's said, person C said " datoyo." I don't understand it. Da to yo is used to quote things but why would you quote person A if person B heard him? Could it simply be that person C is agreeing with what person A said? Like saying " yeah, what he said. " to avoid repeating it?
Assuming C said だとよ to B, and assuming everyone is at the same place, yes, C is basically "quoting" A's statement. Its purpose is to confirm B heard what A said, and to prompt B's response. It's something like "So that's what A is saying, huh?", "(B,) you heard what A said?", "(B,) what do you say after A said this?", etc. Note that saying だとよ by itself does not necessary mean C has agreed with A.