She should first have her school certificates from Sweden recognised. There are offices for this in each federal state of Germany. If you let me know which state she lives in, I can find out which office is responsible.
As she is 19, she is no longer able to attend regular German schools. There are, however, many options for adults to complete their secondary education in Germany. If she completed the 11th grade, then presumably she would be assessed as having reached the level equivalent to a German ‘Realschulabschluss’. This is given after ten years of school education in Germany. With this she can then apply to do an adult education course to complete her education to the level of ‘Abitur’, which is the highest German school leaving certificate and is used for admittance to university.
Throughout Germany, there are adult education schools which offer the chance to do the Abitur. These are commonly called ‘Abendschule / Abendgymnasium / Berufschule / Kolleg’. Some offer the course only in the evening, some are only for people who are working. Again, if I know the part of Germany she is in I can **** for the suitable local one for her.
In addition to having her education from Sweden recognised, she will also have to speak German to a certain level in order to take the course. The most common way to do this is via an integration course. Here is more about it: