A woman made ablution, then put henna over her head (she dyed her hair with henna), then she stood up for her prayers. Is her prayer correct, or not?
And if it invalidates her ablution, should she wipe over the henna, or wash her hair, then make minor ablution for prayer?
Placing henna on the head does not invalidate the state of purity, if she has already performed it. If she performs ablution while there is henna or a similar preparation which women use on their heads, then there is no objection to her wiping over it, during minor ablution (Wudhu).
But in the case of major ablution (Ghusl), she must pour water over it three times, and wiping is not sufficient, according to the authentic Hadith in Sahih Muslim, on the authority of Umm Salamah (may Allah be pleased with her) in which she said: O Messenger of Allah! I tie up my hair; do I need to let it down in order to make ghusl due to being in a state of impurity?
لاَ، إِنَّماَ يَكْفِيْكِ أَنْ تَحْثِيَ عَلَيْهِ ثََلاَثَ حَثَيَاتٍ، ثُمَّ تُفِيْضِيْنَ عَلَيْكِ الْمَاءَ فَتَطْهُرِيْنَ
No, it is enough for you to splash it three times, then pour water over yourself and you will be purified. [Muslim no. 330]
And if you let it down in the case of washing after menstruation, it is better, based upon the other Hadiths which have been narrated concerning this. And Allah is the granter of success.