Hi,
Not so severe but DS2 has low mood and low self esteem. I've spent a lot of time chatting with him about how to be his own best friend.
It's a proper strategy, used in CBT and self-care programmes etc. She has to live with herself. She has no choice in that. So the best decision she can ever make in her entire life, about anything, ever, is to be kind and supportive to herself. To be her own closest friend.
She doesn't have to want to, or to believe it, but if she starts with small acts of self care - having a shower, going for a walk, watching a funny sitcom, listening to uplifting music etc and after each one she gives herself a pat on the back, and acknowledges that she's making progress, then gradually it helps shift the mindset towards a more positive one.
She needs to learn that she must rely on herself to meet her own emotional needs. That you and her boyfriend can support them, but you can't provide them.
You could also try and help her widen her focus. There's a brilliant chapter in the classic self help book 'Feel the Fear' where she says to draw a square and divide it into 9 smaller squares, then write one aspect of life in each square e.g. health; family; romance; study etc, until you have nine areas of interest. The rule is that one must be 'community'and another 'self-development' or self care. The others are whatever you want.
Then you make it a game to try and make all nine areas of equal importance in your life by focusing on them for a chunk of each day. That way, if one aspect of your life goes pear shaped (boyfriend chucks you or you drop out of college) the others help you through the tough time and remind you that some things in life are working out OK.
She does need to realise that manipulating her boyfriend is really not acceptable and will cause them both problems. He can't handle that level of pressure if he's young himself. (I know this from experience. I had a relationship in which I was treated as primary emotional support for a severely depressed friend - even by the MH teams, until I was on the brink of a breakdown myself, as I was a teenager and way out of my depth.)
Hope this helps a bit. PM me if you want to chat more.