Such bizarre logic from people on this subject.
People saying they came first and therefore their needs come first? Do you say this about full siblings? Is the world organised into a hierarchy with the oldest person being the most important, and the youngest the least? He made the choice to have all the children. If you're not willing to share the attention you give to your first children then you should not have more.
Similarly, saying they had no choice about him having another child and therefore should get to demand he spends literally no time with them is so bizarre, does an older sibling with parents who are still together get to take one parent away every weekend? Because they didn't have a choice either.
No children get to choose whether they have siblings. The parents do. They cannot be forced to like them, but they don't get to dictate that their father doesn't have any time with them (which he doesn't if it's every weekend and he works full time and comes home around/past baby's bedtime). And even if they do stomp their feet and try to demand this, no parent should ever consider saying yes to it.
Parents have an obligation to care for all of their children equally. If he comes home past bedtime on the weekdays, then weekends are his only time with his children and it needs to be divided equally or shared by all.
Also, if his daughter is an adult and has moved out then it's not really appropriate for him to leave his partner and baby EVERY weekend to stay with her, just as it wouldn't be appropriate for anyone to visit family every weekend alone when they have a partner and child at home. It's absolutely ridiculous to argue that an adult child is being abandoned by not having her father to stay every weekend, but an 8 month old isn't being abandoned by spending no time at all with their father. When we move out as adults we accept that we won't spend all of our time with our parents anymore, without seeing it as "abandonment".
He has a partner and baby child, his other children have a choice in whether they want a relationship with those people but they should NOT have the choice of whether he does.
People are so incredibly biased on this subject towards the "first" children it blows my mind.