First of all, your GF's friend doesn't decide what your girlfriend does, she decides what she does. Clearly she found something in this friend that she likes, and it's a good thing for her to go and blow of steam with friends. Just because this other girl is a party animal and casual dater doesn't mean your girlfriend is exactly the same. They probably have a lot more in common than you think. Since you can't tell her who she can, and can't be friends with, you should address the issue of the lack of quality time you have together. How often do you see your girlfriend? 2-3 times a week? Is that not enough time for you to feel secure in your connection together? If it's not, you need to bring it up to your girlfriend to let her know how you are feeling. Bring it up at a neutral time, use "I feel" statements, and non-confrontational language that doesn't come across as judgmental or critical of her. Don't even mention the friend. Talk about how you would like to spend more quality time together with her because you enjoy feeling close to her, and that being together and sharing quality time is something you enjoy about your relationship. Tell her how you feel and what you need, and then ask her how she feels and what she needs from you. Then go from there.
"Caring is not an advantage."