I think I can shed some light to this having living in Saudi many years ago and understand the religion deeper
Let's break it in two parts
If she is a Muslimah and she is living under one roof with a Non Mahram which is not a family member be it in separate rooms or not, there will be times inter mingling occurs and there are times opposite sex will come into context hence it is illegal and in the eyes of the Quran and Hadith is haram.
AS one of the hadeeth from Muhammad PBUH said " If there are two persons in a room whom are not mahram, the third person is the Satan."
It does not matter whether it is applied in Saudi , UAE or even Malaysia which is equivalent to khalwat as what SMS mentioned. Punishment varies from different countries to another
In Saudi. Malaysia, Indonesia , Brunei, UAE , men got jail because of this
http://www.witness-pioneer.org/vil/hade ... hap291.htm
Now in Singapore because the context of the governing government which display the country as secular hence the implementation of this ruling on Muslim befalls onto MUIS. The respect of the four main religion i.e Buddhism, Christianity, Hinduism and Islam does not equate of having the power and the teeth to implement Shariah Law per se. MUIS only provide the guidance and information but the implementation is left to the government
Therefore in SG Context it is not illegal as you are not breaking the law of the land. As a Muslim or Muslimah yes you are and is left between you and Allah to decide. MUIS will not take any action to your agenda
BTW I studied the Quran and the Hadeeth during my time in Saudi as there are not much thing to do other than working.

The positive thinker sees the invisible, feels the intangible, and achieves the impossible.Yahoo !!!