It is not permissible for her to get married to a person other than her husband as long she is still his wife. If he dies or divorces her, and her waiting period elapses, it will be permissible for her to marry another man. Allah (Exalted be He) says about the women unlawful for the person to marry:
Forbidden to you (for marriage) are: your mothers, your daughters
to His saying:
Also (forbidden are) women already married, except those (slaves) whom your right hands possess. Thus has Allâh ordained for you. All others are lawful
(Part No. 18; Page No. 225) Allah (Exalted be He) also says,
And those of you who die and leave wives behind them, they (the wives) shall wait (as regards their marriage) for four months and ten (days)
O Prophet (صلى الله عليه وسلم)! When you divorce women, divorce them at their ‘Iddah (prescribed periods) and count (accurately) their ‘Iddah (periods).
To His saying:
Lodge them (the divorced women) where you dwell, according to your means
Moreover, there are many authentic Hadith that explain this matter well. With regard to divorce, her husband may divorce her willingly or she can resort to the legal court to file a divorce. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.