It depends if there are any other 13 YO's there. My DD (17) and her friend that goes along with us (also 17) love it there and usually make friends easily.
But if your friend's DD is kind of sheltered and if the kids there are older teens (16 & 17), she may have a difficult time. From my DD's viewpoint, teens kind of segregate themselves into school grade groups rather than age groups. In other words, some 13 YO's are still in 8th grade, some are HS freshmen, etc.
My DD would probably not want to hang out with an 8th grader or a HS freshman, or probably not even sophomore, for that matter. Teens like to hang around someone who is "old" for their age, rather than "young" for their age.
So, it will depend on who else is in that group and how easily she makes friends. When our DD was 13, and we were on our first cruise, we did not take a friend along and that was a BIG mistake. Our DD would NOT make friends and she hung around us all the time and was miserable. The 2nd cruise, she took along a friend, and between what they did on their own on the ship and Common Grounds, we hardly ever saw them, except for dinner! It was the best of both worlds for all of us -- vacation with the family, but each had time of their own.
If she's able to bring along a friend, it's worth it! Good luck and tell her to have a wonderful time!