Is a woman's ablution valid if henna prevents water from reaching only the front of her head?

If henna forms an insulating layer on the hair that prevents water from reaching it, then ablution (wudu') is not valid with it, and it must be removed according to the majority of scholars who hold the view that wiping the entire head is obligatory. This opinion is more cautious. As for those who permit wiping only part of the head, they see no harm in it. However, if the henna does not form an insulating layer, then it is permissible to wipe over it.