The simple answer is if there are strands of hair within the hairstyle that are still natural try concentrating on selecting these as your target and your colour result would be great. ...More >
Yet if your friends highlights are almost a full head colour it is trickier.
Any colour you apply would be lighter/brighter due to the underlying tone.
I would recommend adding some natural base to you chosen colour to help prevent the brightening effect/ or you could pre-pigment the strands selected for highlighting, before going over with your selected shade. This will add pigment into the highlighted hair to prevent colour bounce or over brightness.