The first thing she should do is start collecting shed hair for the necessary strand testing. She will need to do a certain amount of experimenting in order to get the right mix for her. A quicke guide to the sort of colors that are possible is here, but it is only a guide - the end result depend on the existing color of her hair.
Another thing to keep in mind, is that the color she has when she rinses the mud off, is not the final color - henna and indigo can take up to a week to oxidize - so when she has decided on a mixture, she won't be too shocked and run off to a salon before the color has settled.
Making a henna mix is more an art than a science - personally I use only henna (Purity henna from H4H), on hair that is originally dark blonde, so I can't really comment too much on the indigo/katam admixture aspect. and both the avatar pic and the siggy pic is my hair, from pics taken this spring/summer. A henndigo mix would give a darker and less red color.
Before your mother puts the henndigo mud on her hair, make sure that she has done her research, done her tests, and understands that if she runs to the salon to bleach out the henndigo color, that is likely to end in tears and disaster.
That said, I hope she will figure out a good mixture for her hair, and I also hope she will be happy with the result!
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