Does anyone know what her options are? Is she able to get some sort of medical waiver?

The law is the Education and Skills Act 2008. This is explained on the gov website quite nicely -

You must then do one of the following until you’re 18:

• stay in full-time education, for example at a college

• start an apprenticeship or traineeship

• spend 20 hours or more a week working or volunteering, while in part-time education or training

So, she can drop out and get a part-time job somewhere until her mental health is in a better state.

However, it's also worth her speaking to the local council (who are responsible for attendance) to discuss options.