Is there any heter for a once married woman to stop covering her hair once she is divorced? Whether married for 1 month or 1 year?
Even after a short marriage, a divorced woman must cover her hair.
Yet, the halachah concerning a divorced or widowed woman is less severe than the halachah concerning a married woman, and under certain circumstances there might be room for leniency. This would require knowing the specific case in greater detail.
With best wishes for every success.
See Even Ha’ezer 21:2, and Beis Shmuel 5 (see also Perishah, Chelkas Mechokek; Be’er Heitev); see Iggros Moshe, Even Ha’ezer I, no. 57, who explains that the status of a divorcee or widow remains less stringent than that of a married woman, and permits leaving hair uncovered where covering hair will involve a substantial loss.