While I agree that a GP often can' t help diagnose or treat psychiatric symptoms, some of that sounds like how I was before I was diagnosed with a chronic hormonal disorder. I had intermittent anxiety and depression issues though my teens and twenties, and a large element of it had to do with an underlying medical condition that sadly wasn't diagnosed until I was almost 30.
Hormones can really affect mood a great deal, so you might suggest that she get a basic blood screening for all reproductive hormone levels (and I mean ALL, not just estrogen and progesterone), insulin, blood sugar, thyroid, etc. I have PCOS, which is a syndrome somewhat similar to diabetes, where poor processing of sugar feeds back into abnormal reproductive hormone levels. It most DEFINITELY affected my mood. Not saying she is likely to have that unless she has physical symptoms associated with it, but it is worth ruling out a physical cause first.
Even today, with my PCOS very stable and controlled, I find my mental state easily slips into the bad anxiety/depression patterns if I don't eat a clean, low carb, low sugar diet; exercise regularly; get plenty of sun/natural light.
Also, as a side note, hormonal birth control can also strongly affect mood, both positively and negatively. My depression-free, healthy-lifestyle sister was a basket case on Ortho Tri Cyclene, thought she was going insane for a couple years. Finally I convinced her to go off it for a while, and BOOM, instantly back to normal. She subsequently found a pill brand/type (there's tons of different ones now) that didn't affect her negatively.
Anyway, something for her friend to consider. Sometimes the simplest fixes (underlying condition, medication side effects, diet) get overlooked as the first things to test.