The reason she can't have friends over most of the time isn't her parents fault. She's adopted. That's all I feel comfortable saying.
It sounds like she might be either a victim of perhaps--discrimination by the parents of her friends or there is something more serious you are not communicating.
Is it--she CAN'T have friends over or friends WON'T come over.
B/c it sounds like you are her friend and her being adopted should have no weight on that.
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IF she is truly depressed, lay people cannot help. We might be able to soften a bit by doing nice things, but it will not heal her. Depressed people can't be helped by the world praising them. There is something within them that blocks being able to believe they are capable of much more.
I believe the things that would help--you will only tell us that she isn't able to do them.
In your original post it really sounds as though her parents are the one blocking all of this.
And no, I'm sorry, being adopted is not a legitimate reason.
A youtube video isn't going to fix this.
Her having the love and support of family and friends will go much further.
BTDT myself. Teenagers especially are ill-equipped to recognize the proper signs of when help is needed. We just do not have the life experience, nor skills to recognize issues.
If you want to help--be her friend--unconditionally!