If the woman will come out to non mahram men and show them her henna decoration, it is not permissible to help her do that. Kindly refer to the following link(s):   https://www.assimalhakeem.net/is-allowed-for-a-muslim-women-to-learn-headdresscoiffure-and-to-work-as-a-hairdresser-in-a-nonmuslim-countrywhat-about-cosmetic

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Yes, provided you cover it in front of non mahram men.

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Dreadlocks are made of using one own’s hair by twisting and locking them and this is permissible. In case of wudhu, wiping over the head is sufficient, while for ghusl, the water must reach the roots of the hair and your scalp. You say the water reaches the scalp / roots, so this is permissible.

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