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

Is it haram for a female to do make up/henna on other females. I know there’s a Hadith stating that, the one who sharpens her teeth, the one who wears fuax lashes, the one who wears faux hair and also the one who applies to the female are cursed. However if she avoids these is it still haram to beautify women for weddings etc…