Home Hairdressing 'Ombre onto black hair'

Hi guys!!!
I'm not a qualified hair dresser but I do cut and dye my friends/family's hair here in Australia and have done for a long time as they charge a bomb here for hair. I recently done ombre on my friends hair and it turned out beauts. Another friend of mine wants me to do a red ombre on her hair and I'm pretty confident to do to it, but only thing is, she has near enough black hair, and her roots need doing. She actually wanted all her hair dyed completely black, but I know it will be extremely difficult to do the ombre on her hair after its been dyed black and can damage it. Am I best to just do her roots then dry off her hair then bleach to start the ombre? Or should I dye her hair black leave her with black hair for a few days then come back for the ombre?

Many thanks