Get the henna out first. Back in the 80's (yeah, I'm old) when everyone was into henna, a hairdresser friend told me two good ways to get out henna. 1) Shampoo a couple times with Prell, the old-school thick green shampoo. You may be able to find it in a local non-chain drugstore. 2) Pectin. It's in the grocery store in the aisle with sugar and jelly making supplies. Pack your hair with it, let it sit for 15 to 20 minutes, then wash it out.