It's not easy to overcome, but exercise can help as part of the process.
Have they had their cortisol levels checked? When serotonin levels are low, depression as well as insomnia and anxiety can occur. Cortisol can "burn out" so it's worth getting this tested by a doctor.

In the meantime try exercise.

They say magnesium and vitamin d3 can help depression. But your friend should ALWAYS consult a doctor first before taking these.