She works in a residence for foreign female students

Firstly: 

There is nothing wrong with your sister working in the
students’ residence, especially if it is in your city, but if it is in
another city then it is not permissible for her to travel there and back
except with a mahram, but it is permissible for her to stay in that
residence without a mahram, because what is required is only that she be
accompanied by the mahram on the road. The same also applies to the female
students who live there. Your sister is not expected to examine each student
to find out whether she came with her mahram or travelled without him,
rather it is sufficient that her work in the residence does not involve
anything haraam. Administration, guard duties, cleaning and so on are all
permissible and there is nothing wrong with that, in sha Allaah. 

Shaykh Ibn Baaz (may Allaah have mercy on him) said in
Fataawa Noor ‘ala al-Darb (tape no. 288): 

There is nothing wrong with what you have done. There is no
harm in a woman staying or living in a city without a mahram and there is no
sin in it, especially if there is no danger involved in that, if she is
among other women or in a workplace that is separate from men doing work
that Allaah has permitted, or in a department that is women-only. There is
nothing wrong with any of that. What is forbidden is travelling; you should
not travel except with a mahram, and you should not come without a mahram.
As for staying among women doing permissible work, there is nothing wrong
with that, praise be to Allaah. End quote. 

See also the answer to question no.
45917. 

Secondly: 

With regard to the hadeeth mentioned at the end of the
question, it is the hadeeth of ‘Aa’ishah according to which the Prophet
(peace and blessings of Allaah be upon him) said: “There is no woman who
takes off her clothes anywhere but in her own house, but she has torn (the
veil) which exists between her and Allaah.” Narrated by Abu Dawood (4010)
and classed as saheeh by al-Albaani. 

Al-Manaawi said in Fayd al-Qadeer (3/163), commenting
on this hadeeth: 

It is a metaphor for her uncovering herself before
non-mahrams and not covering herself in front of them. End quote. 

What is meant in this hadeeth is a woman who takes off her
clothes in the presence of non-mahram men, or in a place where non-mahram
man can see her, such as public places or places frequented by evil-doers
and the like. But if she is in a safe place, such as if she is in the house
of her mahrams or parents, or in a place where she and her husband have made
a stop during a journey, or she needs medical treatment or she needs to do
ghusl for some shar’i reason and so on, then there is no sin on her, and the
hadeeth does not forbid such cases. 

This has also been explained in the answer to question no.
34750. 

Thirdly: 

The sincere Muslim carries his da’wah with him wherever he
goes. If the situation in the students’ residence is as you describe, and
there is little righteousness, then your sister should strive to spread
goodness there and enjoin what is good and forbid what is evil, and strive
to change things for the better as much as she can. Thus she will fulfil her
duty before Allaah. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked the
following question in Majmoo’ al-Fataawa (7/274): 

I am a girl who lives in the students’ residence. Allaah has
guided me to the truth, and I have begun to adhere to it, praise be to
Allaah. But I am very upset because of what I see around me of some sins and
evil deeds, especially among some of my fellow-students, such as listening
to music and malicious gossip. I advised them often but some of them make
fun of me and mock me, and say that I am being extreme. What should I do,
may Allaah reward you with good? 

He (may Allaah have mercy on him) replied: 

What you have to do is denounce the evil as much as you can,
by kind words and good manners, quoting verses and ahaadeeth which speak of
that, according to your level of knowledge. Do not take part in listening to
music or gossiping or other haraam words and deeds, and withdraw as much as
you can until they start to speak about something else, because Allaah says
(interpretation of the meaning): 

“And when you (Muhammad) see those who engage in a false
conversation about Our Verses (of the Qur’aan) by mocking at them, stay away
from them till they turn to another topic”

[al-An’aam 6:68]

If you denounce them as much as you can and withdraw from
what they are doing, then their deeds or criticism of you will not affect
you, as Allaah says (interpretation of the meaning): 

“O you who believe! Take care of your ownselves. If you
follow the (right) guidance [and enjoin what is right (Islamic Monotheism
and all that Islam orders one to do) and forbid what is wrong (polytheism,
disbelief and all that Islam has forbidden)] no hurt can come to you from
those who are in error. The return of you all is to Allaah, then He will
inform you about (all) that which you used to do”

[al-Maa’idah 5:105]

Allaah explains that the believer will not be harmed by those
who go astray if he adheres to the truth and follows true guidance, by
denouncing evil and adhering steadfastly to the truth and calling others to
it. Allaah will grant you a way out, and Allaah will benefit them by means
of your guidance if you are patient and seek reward, in sha Allaah. Receive
the glad tidings of great goodness and good consequences so long as you
remain steadfast in adhering to the truth and denouncing that which goes
against it, as Allaah says (interpretation of the meaning): 

“and the (blessed) end is for the Muttaqoon (the pious)”

[al-A’raaf 7:128] 

“So be patient. Surely, the (good) end is for the
Muttaqoon (the pious)”

[Hood 11:49] 

“As for those who strive hard in Us (Our Cause), We will
surely guide them to Our paths (i.e. Allaah’s religion — Islamic
Monotheism). And verily, Allaah is with the Muhsinoon (good-doers)”

[al-‘Ankaboot 29:69]

May Allaah help you to do that which pleases Him and bless
you with patience and steadfastness, and may He help your sisters and family
and colleagues to do that which He loves and which pleases Him, for He is
All-Hearing, Ever-Near, and He is the Guide to the straight path. End
quote. 

And Allaah knows best.