anyone in a relationship is entitled to say they don't want any/any more children and that's fine
I completely disagree with this.
As always, though, context matters. Ideally, the conversation about having DC takes place before getting married / being long-term committed. If one person definitely doesn't want kids, that's a clear decision for the other to make.
Of course, no one can really predict that they won't change their mind, want fewer or more DC than they might have first said. That can happen.
What I disagree with is the notion someone can baldly state it, and that's it - no discussion, no consideration of how the other person (that you love!) feels. One party cannot pressurise another to have a child of course, but stating 'no' with no openness to hearing your partner's perspective is not reasonable nor how relationships should work.
I know plenty of couples who have had to discuss having a 3rd or 4th baby. One wants it, other less sure. I know cases where couples have gone ahead, even with one more keen than the other, and where they haven't, with one more disappointed than the other
In OP's case, it's even more difficult, given the disabilities their DC has, and all the ramifications of caring for them & having another DC. It's hard for both, but even more reason for a proper discussion, done respectfully.