BlackMasterPiece, I transitioned my mom to henna as well. Similar story, chemical dyes = disaster! She is 99% gray. She likes the medium-dark reddish brownish look (similar to your mom's roots in the pics above).
When I went home in Nov. I used 50 grams of henna to 25 grams of indigo. TOO RED! Kinda orangey/red... but we didn't want it too dark, so I started out with less indigo at first.
When I went back for Christmas I used 50 grams of henna to 50 grams of indigo. PERFECT!
If your mom wants to maintain the reddish color already there on the rest of her hair, I would advise you to only put the henna/indigo mix on the gray newgrowth and just put henna only on the rest of the hair. In my experience, putting the henna/indigo mix on gray hair is very different than putting it on darker hair. It may darken the previously colored hair more than you may want.
Mix up the henna at least 4 hours before you use it. I put a splash of lemon juice in it to help w/ color release. If you put a little piece of a white paper towel on top of the mix, it will turn red when the dye releases. You can also look at it and see little red puddles.
Mix up the indigo (w/ a pinch of sea salt) after mom is in the chair and prepped. Indigo releases quickly and needs to be applied directly after mixing (but not before mixing it with the henna).
I keep the mix on mom's head for at least 4 hours.
I also have gray in the top of my head and do the same thing for myself. I made the mistake of putting the henna/indigo mix on my entire head and it darkened my hair. So glad I didn't do that to mom's head. She would not let me back in her head. I'll survive, but her precious reddish brown color is a must-have for her.
Hope This Helps!