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”
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.
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.