Can she take off her Hijab in front of people who have learning difficulties?

Firstly: 

It is obligatory for a woman to cover all of her body in
front of non-mahram men. See the answer to question no.
11774. 

She should not speak to non-mahram men except in the case of
need, without softening her voice, as Allah says (interpretation of the
meaning):

“then be not soft in speech, lest he in whose heart is a
disease (of hypocrisy, or evil desire for adultery) should be moved with
desire, but speak in an honourable manner”

[al-Ahzaab 33:32]. 

Secondly: 

The basic principle is that it is obligatory to wear Hijab in
front of all non-mahram men. That includes those who have learning
difficulties, except those who have developmental delays which mean that
they do not understand about women and have no inclination or desire towards
them. Such a person comes under the same heading as “old male servants
who lack vigour” [al-Noor 24:31]. So a woman may uncover in front of him
what she in covers in front of her mahram, which is usually what appears of
her, such as the head, face, forearms and feet. 

But if he understands about women and feels inclination
towards them, then he is like any other man and she must observe hijab in
front of him.

Ibn Qudaamah (may Allah have mercy on him)
said: Whoever no longer feels any desire, because of old age, impotence or
incurable illness, or because he is a eunuch, … or a mukhannath (the
effeminate man or a man who has female hormones) who feels no desire, the
ruling that applies to such a man is the same as the ruling that applies to
mahrams regarding looking at women, because Allaah says (interpretation of
the meaning): ‘… or old male servants who lack vigour…’ [al-Noor 24:31],
i.e., those who feel no desire for women.” Ibn ‘Abbaas said: “This is the
one of whom women do not feel shy.” He also said: “This is the mukhannath
who is impotent (i.e., cannot have an erection).”

It was reported that Mujaahid and Qutaadah
said: “This is the one who has no interest in women, but if he is a
mukhannath who feels desire and knows about women, then the rules that apply
to others apply also to him, because ‘Aa’ishah said: ‘A mukhannath entered
upon the wives of the Prophet (peace and blessings of Allaah be upon him),
and they used to think that he was a man who felt no physical desires, but
the Prophet (peace and blessings of Allaah be upon him) entered upon us when
this man was describing a woman and saying ‘When she comes in, she comes on
four, and when she goes out, she goes on eight.’ The Prophet (peace and
blessings of Allaah be upon him) said: ‘Do I not see that this man knows who
is here? This one should never enter upon you.’ And he was kept away after
that.” (Reported by Abu Dawood and others).

End quote from al-Mughni, 7/463 

Shaykh Ibn Jibreen (may Allah have mercy on him) was asked
about an adult person who is developmentally delayed: do women have to
observe hijab in front of him? 

He replied: If the developmental delay is severe, in the
sense that he cannot think or understand, and he does not know what things
mean and he has no desire that may prompt him to look and touch and so on,
and he has no interest in women, rather he is like a child or less than
that, then there is no need to observe hijab in front of him, and he is
included in the words of Allah (interpretation of the meaning) “old male
servants who lack vigour” [al-Noor 24:31]. But if he understands some of
these matters and he has an inclination towards women, and it is clear from
what he says that he feels desire, then he should not be allowed to enter
upon women and they have to observe hijab in front of him, because of the
story of the mukhannath who said to the brother of Umm Salamah: If you
conquer al-Taa’if, I will show you the daughter of Ghaylaan, for she comes
on four (folds of fat), and goes on eight. The Prophet (blessings and peace
of Allah be upon him) said: “I think that this man knows what’s what. This
one should never enter upon you.” Narrated by al-Bukhaari and others. And
Allah knows best. 

End quote from Fataawah al-Shaykh Ibn Jibreen. 

Thirdly:  

Attention must be paid to the prohibition on being alone with
this non-mahram who is of sound mind; rather attention should be paid to the
prohibition on working in mixed environments. Please see the answer the
questions no. 45905 and
1200. 

We ask Allah to help and guide you. 

And Allah knows best.