If she is on medication, and it doesn't seem to be working, talking to her doctor or therapist might actually help her out more than anything you could try to do. They could prescribe her another medication or talk to her and see if there are any other issues going on that could be a cause of her continued depression, aside from any other known problems.
Ingesting herbs, while on certain medications, can actually interfere with the medication. I wouldn't tell her to consume lavender, without first talking to her doctor, just to be safe. Lavender bubble bath, lotions, oils, and so on should be ok. She could add some of the essential oil to her bath and then use the lotion afterwards. Lavender is very calming and soothing, so it could help her and calm her.
Onto the stones. I personally recommend lace agate. They are supposed to improve problems with depression, and energy flow. White lace agate and pink lace agate are some good options. The white lace improves optimism and mood. The pink promotes happiness.
Moonstone is associated with attunement with the goddess, balancing female hormones, calming, intuition, psychic abilities, protection, love, and divination.
Amethyst is usually used for work with the third eye, psychic awareness, and so on. But, I also read that it can help with inner peace, meditation, pain of loss, and promotion of happiness. I've only ever used it for work in divination, but you could try it for the other properties and see how it works.
I added a few sites, below, that some more information on gemstones and their properties.