Don’t use any oxidising colour inc. CT. or any tint that requires mixing with a developer.
Most direct dye semi’s should be ok.
You need a chelating shampoo not a clarifying shampoo. They’re different. A chelating shampoo will remove mineral deposits such as lime but she might need several goes and it’s very drying, so I’d recommend doing it over a period of time. Theres also Malibu C crystal gel that comes in sachets that’s very effective. Also, check out shampoos marketed for swimmers to remove chlorine build up.
You‘ll need to take test cuttings from different areas before colouring as it might still be present in some parts of the hair, especially if there’s a lot of hair.
Do you know what’s caused the mineral deposits? If it’s a colour product the client has used, then providing it’s not used again, it should eventually wash out. Some henna’s can take months to remove (!). Could it be caused by the water supply? I had a client that had to shampoo her hair using bottled water because she had heavy minerals in her well water supplying the house.