What an interesting one!! I do see his reservations when it comes to whether or not you could ever come first with someone who has children. It's not a brilliant thought.. and a single female friend of mine says the same about a guy with any children.
However, it's easy to see things as black and white when it's just a principle/idea. Not so easy if you meet someone you think is just fab, and they happen to have a child/children.
I met my DH online. We didn't meet all of each other's set criteria at all (beware writing a shopping list, you could be doing yourself out of all kinds of great experiences and relationships!!) Not sure what he thought of the kids thing. However, we worked it out! It's up to the adults involved to make it work for them. It also helps that neither of us are that into 'going out' dating/pubs/clubs etc. We both really like the family lifestyle, into similar food/telly etc. Some basic compatibilities!
Also, the parent in the relationship has to (over time and built-up trust, obv) allow the incoming partner at least SOME rights as a significant adult in their child's life, IMHO. Otherwise, as the incoming partner, you will have a few responsibilities there but nothing to balance it out. Makes building a relationship with that child very difficult for them, too, which has ended up being totally central to any success we've had. It helped that DD's bio left long ago, so as far as she was concerned, there was a vacancy. She interviewed and after some time offered new partner a position! So it felt like her choice too, to her.
He was mature enough at the beginning to realise what's involved (tho no kids himself at the time) and be open to it. I saw it as a sign of maturity in him, too, though he's a few years younger than me.
Also helped that I didn't need rescuing, I had a great life and DD was very secure, I just liked the idea of someone to share things with. The fact that DD and I were confident and secure individuals who had worked through any baggage as much as we could helped too. We were not looking for a walking wallet, or any other thing of the sort. And I was prepared to make space for a new partner emotionally, not simply harp on about my child always coming first. It's not that clear-cut, they have ended up coming equal first. I've caught myself stepping in a few times if I feel he's made the wrong judgement call, but we've discussed it afterwards and always find a way to back each other up in the end.
Not all single parents are hopelessly under-resourced in every way! I would't want to be a rescuer either.
Show your friend this threat!!! 