This is not permissible as this gives the impression that the hair is sticker and longer than it really is and this falls under hair extensions.

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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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