My mother in law has a question , she started wearing hijab when her son was in hospital she said that she took a vow to wear it so God would save her son's life . But now that she is older past the age of menses and is not seeking a husband she wants to know if she can take it off because it is difficult for her to wear it when it is very hot outside due to hormonal changes

Hijab is obligatory for the Muslim adult woman and it continues to be as such even after she reaches menopause. Your mother-in-law can choose type of tissues that suits her case.