So far my best bets have been talking about the Demographic Transition Model - i.e, as wealth goes up, birth rates decline, so uplift the poor and we'll be fine.
I also say that we have enough resources to go around if we stop allowing 1 man to use the resources of 100.
Theres also people with mad conspiracies that the government is trying to decrease birth rates by promoting LGBT stuff. To that I've said I also say that the current system of capitalism is all about growth and making money, and more people = more money. They don't want to thin us down.
What else can I say? It seems to be a very common belief.
I never met them IRL but on the internet it's simple enough to just say "you first".
They're the kind of person to complain about traffic while sitting in traffic. You are the traffic.
Just press them on exactly who they think is overpopulated and why.
Lol, I always go with that too. "I agree, we should start with white pmc libs, they're the biggest polluters" and then they get mad.