Op has said how much her childcare costs, and it's not 2k is it? It's less than £1000. So in this situation, which is what we're talking about, he's paying it all*
If OP's "friend" is lucky enough to live somewhere where full time childcare costs £1k per month (well below average per ONS so unusual) then the father's share of that is £500. The share of the increased housing costs to rent/ buy somewhere larger so that there is space for the child would likely take up the other £500, compared to a studio/ house share suitable for a single person.
The child then needs food, clothes, birthday/ Christmas presents, some opportunity to join in social activities/ clubs/ birthday parties if friends, to be able to use public transport/ a car, school uniform and trips, furnishings for their bedroom, perhaps even (shock horror!) should be able to expect the occasional holiday considering they have a parent earning a six figure salary. They also need heating, water, etc. The amount paid would contribute little/ nothing towards that once childcare and increased housing costs for a suitable place for a child to live are contributed to, even in the cheapest kind of area in your hypothetical situation.
And are we to imagine that said six figure salary earning parent was living in such an area before doing his vanishing act and abandoning his child and the mother of his child? Or that he will be living that kind of life with his £5k net per month as a single man after he pays his proportionately paltry maintenance? Not good enough for him I'm sure but for his child apparently it would be, in your imagined scenario. While the mother's career progression suffers as she tries to not only cover more than her fair share of costs for the child but also provide all of their care.
As I said earlier, it's very depressing that women are defending men who behave this way and are happy to run off leaving a woman and child in this situation, and expect their child to grow up with a lower standard of living while maintaining their own.