Agree with othwrs to go with the things she is already interested in and support and celebrate that first, most importantly. Then, there are a number of sports for which height is an advantage (rowing springs to mind, also women's rugby, cricket, tennis...lots of ithers!) Lots of people discover these sports late and go on to excel in them, 15 is relatively young. Regular attendance to sports clubs, building skills and then helping newer arrivals is a great way to build lots of new relationships of all ages.
Or, how about a bit of part time work. At 15 she can do a couple of hours a week. If there is a local internet cafe that could be a place where she would feel in her element, or perhaps volunteering at the local library?
Or would she like to take up learning an instrument? Piano, cello, clarinet/bassoon? There could be local youth orchestras taking beginners, and community ones she could join once she gets to grade 3/5.
See what there is available locally, too, as starting where you are and finding others who want to meet people is often the quickest way to make connections.
Even local church groups can run interesting summer clubs, and teenagers don't have to be believers to join in, some are designed to be user friendly to people who are not believers. Councils, churches and other religious organisations also run community projects she might be interested in helping out with.