Most professional colorist will tell you not to go more than two shades lighter. So I guess a dark brown, chestnut maybe? Your friend could just put some highlights in. Not a total color change , but it will brighten their look.
Don't do blonde, that is harsh on the hair. Put blue or red or even purple streaks in it but not bright. Those colours turn out nice in the light because they are subtle but noticeable! Highlighting the hair would be good
Hi! If your friend is looking for a change, let's do it subtle like. How about this idea: A Henna. These are environmentally correct, better for the hair itself and won't be a "shocker" when they look in the mirror. a nice color on Black/dark hair is a mahagohony, or raspberry tint. It deepens, shines, their natural color, yet, when they get in the sun it's a brilliant gloss and incredible glow of warmth and satire! They are lots of fun, healthy, and normally are greeted with great response from others. Good Luck! Have fun and remember, it will grow back! Have a nice night!