Over the past decade+ I've also found that an ability to monitor and hack your own mood is an incredibly valuable skill. Know what things trigger depression and either avoid them or work out a contingency plan to weather the storm... Have a mind-killer available for when a spiral is coming and force yourself to use it.

This is basically exactly what I've been working on to overcome my own depression.
Reading Less Wrong and working on the basic techniques of the sequences has made a huge difference. And note that the "mind-killer" doesn't have to be a particularly healthy behavior - it just has to be better than the depression (at least to start on).

It sounds like your girlfriend is dealing with some heavy external stuff, in addition to chemical brain stuff. I've found it helpful to remind myself that that which can be destroyed by the truth should be, but the reverse is also true. For me, being able to feel the difference gives me huge leverage on eliminating the irrational kind.