I'm a big advocate of therapy, but if your buddy isn't into that, an exercise regimen might help, if he's not on one already. Adrenaline can raise testosterone levels in female-bodied persons, and it's helped me personally quite a bit. Fish oil has also been shown to increase testosterone, and the capsules are pretty inexpensive.
The calming effects of testosterone, I have to believe, come with the suppression of female hormones, in some cases, and with the feeling that you're finally living as the person you are, in others. In my case, I suffer from horrible PMDD and debilitating menstrual cramps, as well as bipolar disorder that cycles with my menses; testosterone stopped all that, and made my life exponentially easier. Unforunately, there's not much you can do to suppress female hormones apart from replacing them.
Your friend's moods could be helped quite a bit by therapy, I'm sure, but again, in its absence, it is possible to do some things. B12 deficiency can cause depression, as can Vitamin K deficiency, so multivitamins are always a good idea. I'd steer clear of St. John's Wort, though; it has properties comparable to MAOI antidepressants... which means it has potentially deadly interactions with certain drugs and foods.
Good luck.
SD