When someone starts to bring up Wagner Russonazis I know that I won't get anywhere with them in terms of the big picture of anti-imperialism. There's no real point in further discussion. Russiagate brainworms have been incredibly effective in this regard. Even if they admit that mercenaries are and always have been a part of war, they'll just change their tune to "Putin's personal Nazi PMC" if they haven't already.
You could also just respond with something about Blackwater, or the German military's Nazi infestation. Then they'll call whataboutism and the conversation will be over, but there wasn't any constructive dialog anyway and maybe some spectators (if it's not a private conversation) may turn on their brains a bit and look deeper.