As far as a gang tattoo placement, you are right that the neck is a common spot. But that does not mean that she will look like a gangster simply because of it. Most of her style will be based on the way she carries herself, dresses and the types of clothes that she wears. I've seen many organic or nature inspired tattoos on earthy gals that run clear up their backs and to their necks and you would never think they were in any sort of gang so it really comes down to style.
Decision choices are also going to play a big part of the consideration in a neck tattoo. If you are not already familiar with, be sure to read Five Popular Tattoo Art Styles. Gang style tattoos are often the original prison style tattoos as documented in the movie Tattoo Nation. These designs include fine line black and grey roses, hearts, portraits, stars and of course script tattoos. If she opts for something like feathers , cherry blossoms , a tree tattoo or a dragon for example, her design would not have that traditional look to it, and could rather be quite colorful and interesting. Another feminine option is a permanent Henna style tattoo if she likes delicate and intricate flowers with vines.
Next I'd like to remind your friend to take the time to seek out a reputable tattoo artist for the job. If she already has some ideas, great. If not, a tattoo artist can help her decide her options regarding the tattoo placement. Make sure you look at the artist's portfolio and ALWAYS follow their proper tattoo aftercare instructions.
Because of this large tattooed area, wearing a sunblock with a minimum of SPF 30 is necessary to prevent fading of her tattoo. Especially during summer and swimsuit season where her back and neck will be exposed to the elements.
Neck tattoos are not for everyone. Just like a hand, wrist or face tattoo they are not for the commitment-phobic and they make a bold statement. Hopefully with some time and suitable thought she can find a design that will reflect her personality and without any doubt, add to the individual that she is.
I hope this helps and best of luck!