cassia for white and iron grey hair

Hello,

I use henna and am happy with it. My friend wants to try using cassia. She doesn't want any red or strawberry blond colour. She has white hair and iron grey hair. I know the cassia will colour the white hair. Will it cover the iron grey hair? She doesn't have dye-resistant grey. She wants a blond or light ash blond hair. She'll even go a bit darker; she just doesn't want red/copper/strawberry colour.

I'm not sure what to mix with the cassia, or even if I need to mix henna or indigo with the cassia. I'd appreciate any thoughts!

thanks!