You cannot gain this woman's consent for a termination, therefore you do not force one upon her. Consent is everything when it comes to a termination. You do not forcibly remove a foetus from an unwilling woman.
So, because this woman cannot consent, you force her to go through a pregnancy even if it has the potential to cause her serious, permanent physical or mental injury - even when all concerned are sure that, but for her incapacity, she would consent to a termination in preference to being seriously disabled for the rest of her life. How is that remotely humane?
As for the suggestion that this must be OK because her social worker says so: precisely what mental health qualifications does the social worker have? How well does s/he know the patient?
Irrespective of how intelligent this woman is, how is any human being going to feel about having their wanted child killed when it's in their womb? Perhaps you are just not capable of imagining the reality of what you are suggesting. She could be psychologically scarred for life because of that. I think most women on the planet would find that experience horrifically traumatic
Pouncer, you're making the serious mistake of assuming that this woman would experience a termination in the same way as you might. That requires a deep understanding of what is going on and what is happening, and there is no evidence that this woman has that. We're told that she has little or no concept what having a baby involves.
More seriously, people who know much more about her than you do have assessed that she will find a termination much less upsetting than having the baby taken away once born, or having to leave her home and go into care amongst strangers. How the hell do you claim to know better than they do?