Henna after using box dyes?

I must stress it's important you use body art quality henna. Many hair dye 'box' hennas contain metallic salts that will react badly with peroxide and bleach if your friend was to change their mind and put a dye on top. Henna really can't be removed or bleached out without seriously compromising the hair's integrity so your friend must be ready to commit. I have read of successful removals with ColourB4 and similar products but these are likely flukes as henna works in a completely different way than oxidative dye.

Depending on the natural colour of your friends hair there will probably be a difference in colour between henna on already dyed red hair and henna on virgin regrowth. Henna is basically a colour-glaze, it will not lighten the hair's pigment in any way. On dark hair even the brightest henna will give merely a sheen of colour when the light hits it.

I must stress it's important you use body art quality henna.

And after you dye your hair you can make beautiful patterns)))

Okay, back to the topic.
I used henna after box dyes many times just to fix my corrupted hair. But I had a side efffect - my hair was red despite all colors I applied on it after dyeing with henna.
Now I'm not using nor henna nor box dyes for a year, but my hair is still red. I'm starting to think that it is my natural color 0_o

I just want to add that while henna is translucent, with each application the colour continues to build and darken.  After a few months of using henna I could see no difference in my previously dyed/bleached hair and new growth.  My hair was a wreck after over-bleaching etc. at the time.  Henna is lovely and conditioning and does no harm to your hair.    After you use henna and rinse it off, the colour even continues to "develop" for another day or so.  After a year of using it my hair was a very dark reddish brown.  I've read how some people only touch up the roots and use a diluted henna on the rest and if I ever go back to henna this is what I would do to keep it from going too dark. 
But yes like what others have mentioned, get the body art quality only.  And....its permanent.  I stopped using it a year ago and have used Colour B4 and dyed over the top, but the red is still there!  It's like a fatal attraction - everything is lovely as long as you're together, but if you want to get rid of it...well that's where the happiness ends lol.