Peroxide technically is bleach, but on its own it can lighten, and since her hair isn't a super light color, it'd probably be safer to do that so that you don't go too light. It'll still be drying but not as much.

There are tutorials on youtube, but if you just spray peroxide on your hair and leave it in overnight, and maybe redo it another time or two if you have to, I've seen people get results. also mixing honey with your conditioner, because it's a lightening agent. and the henna you already have should help with the red hue
I'm 15 now but I don't get on often so I may be 67 by the time you read this.
My name is Sarah.
I come on here when I want hair advice because if you type in "curly hair" anywhere, all you get is advice on how to curl it. I used to get on here a lot when I was like, 12 or 13 (and first figuring out how to stop straightening my hair without being bullied), so if you happen to see some older posts of mine, shh. No judging.