Way to completely miss the point.
Nope, you just didn't understand.
Germany guarantees every religion to enact their beliefs freely. That said, you have to belong to an acknowledged religious group to get that right.
Saying "The spaghetti monster forbids!" is not enough as long as the Spaghetti monster isn't an acknowledged religion in Germany.
The Islam is officially acknowledged, therefore the girl has rights, guaranteed by the German constitution.
Since the religion doesn't forbid her to go swimming when properly clothed with that mentioned swimming suit, she doesn't have the right to "not go swimming".
Kafein asked why religious authorities have a say in this. I answered him on point and now even explained it to you.