Praise be to Allah.
She must observe the ‘iddah prescribed in Islamic teachings, and continue to mourn (in the manner prescribed in Islam) for the duration of the entire ‘iddah period. But she may go out by day in order to work, because work comes under the heading of essential needs, and the scholars have stated that it is permissible for a woman who is observing ‘iddah following the death of her husband to go out during the day to attend to her needs. Work is one of the most important of needs, and if she needs to do that by night, it is permissible for her to go out because of necessity, lest she lose her job, and the harmful consequences that may result from losing her job are no secret, if she needs this job. The scholars have stated many reasons that make it permissible for a woman to go out of her husband’s house in which she is required to observe ‘iddah, some of which are less serious than going out to work, if she has no choice but to keep this job. The basic principle concerning that is the verse in which Allah, may He be glorified, says: “So fear Allah as much as you are able” [at-Taghaabun 64:16], and the words of the Prophet (blessings and peace of Allah be upon him): “If I instruct you to do something, then do as much of it as you are able.” Saheeh – agreed upon. And Allah, may He be glorified and exalted) knows best. Ends.
Majmoo‘ Fataawa Ibn Baaz (22/201).