I had counselling for antenatal depression when I was pg. I was having a girl; when I found out I was devastated. I had wanted a boy my whole life. The strength of my feelings of disappointment shocked me.
Of course, looking back, I can see that my brain was riddled with hormones, I would have had the depression regardless of the gender of the baby, but at the time it felt awful. The worst thing was, I was carrying a healthy baby, and knowing how may people in the world would love to be in that position made me feel incredibly guilty for feeling the way I did.
The only information I could access about Gender disappointment was from mothers expecting boys and wanting girls- this made me feel even worse.
Interestingly, my counsellor said that a large proportion of the expectant parents she saw suffering Gender disappointment were those who had been infertile for many years, and had eventually conceived through IVF; they had held on to a dream of a (usually) baby girl for so long, that when they conceived twin boys they were devastated.
It is almost taboo to discuss it- I could never tell anybody how disappointed I was, it seemed so ungrateful.
Of course, now my DD is 2yo and I couldn't imagine loving her more than I do.
Don't be too quick to dismiss someone as ungrateful and selfish for having an irrational reaction to the gender of their child. It's far more complicated than it looks.
Support your friend.