Eh. If it's your friend I think it's fair game to ask. I've had tons of people in my life say they don't want kids and then once they've changed partners, they wanted kids.

I honestly don't think it should be an issue to ask in general if people want kids either. Humans have only made it this far because of our intelligence, curiosity, and reproduction. It's a simple yes or no question.

I think prying or attempting to change the person's mind is the issue. If someone says the don't want kids, that should be the end of it.