I think the OP just came here to vent feelings. Her friend said to her that birth is not natural. Yeah, OP. I feel ya. That seems like a weird statement to me.
As to the pain aspect, her friend says that birth is "SOOO painful" but this friend has not given birth. It's not like we can all say- don't dismiss this friend's personal experience. I never want to dismiss the experience a woman has with her own body and birth. But it does make me sad that this friend, and it seems a lot of people, discount the possibility that birth might NOT be painful. A PP talks about relieving the "suffering" of birth. I had an awesome time with my unmedicated birth. No suffering involved. Not getting an epidural doesn't automatically mean pain and suffering. The sad thing to me is that people don't recognize that.
I'm totally cool with women planning whatever kind of birth they want, but it might make me also feel that they don't know the full range of possibilities if they just assume that birth is automatically the most horrific thing a woman can go through.
So OP, the "mental check" you got is probably a good thing for your friendship but I think I understand what you're feeling.