Ask them what their B12 and iron levels are lol. Ask them what their daily protein intake is. If they are actually so worried about it, they can answer that.
For example. I hit my daily B12 and iron goals every day and just for lunch today, I consumed 40g of protein. I think I was actually lower on carbs than I tend to aim for.
In the words of UlyssesT: I HATE CONCERN TROLLING! I HATE CONCERN TROLLING!
My current battle is with the dumb fucks that try to say that growing plants for vegan diets kill more critters than growing plants for factory farming, when 70% of crops go towards factory farms regardless. You don't even need to know math to figure that one out.