Wait until she hits Peri-menopause in her early 40s
Women are very hormonal and very emotional. They can get depressed and stay depressed and they have no idea of why. My wife went on Celexa for about 2 years. I don't think that was a good solution.
Right now she does the Progesterone cream and I got her some very weak 4-AD sublinguals that will hopefully balance out her hormones. She seems to be a lot less depressed.
I would think your gf is too young to do a self treated HRT.
I think she could try taking some 5-HTP, and some Black Cohosh every day. Have her take a good multi vitamin. Just get her on a regiment of taking these every day. If she is like my wife she will do it for about a week and then stop.