I don’t know if interventions work or not, but have you considered gathering a group of her closest friends for one with her? You, as an individual, can only do so much.

Check this out:
http://www.helpguide.org/mental/bulimia_signs_symptoms_causes_treatment.htm

It sounds like she could be anorexic rather than bulemic. But the advice would be the same.

If you have access to them, talk to her family, her parents. Tell them what you suspect. Then talk to your friend.

Firstly, before talking to your friends family and other loved ones, I think it might be best to talk to her. It’s possible that she’ll reach out. It’s also possible that she’ll become defensive. Then, maybe you could talk to her family and other friends.
Sadly, the most you can do is let her know you’re there for her and ready to support her. The rest is down to her.
She’s the only person that can change things. Knowing she has people around her who love and care for her might urge her to seek help.
Good luck.

I would just like to point out that being in your mid-twenties and writing your memoirs means nothing more than you have lived, you have experiences and you are reflecting on those times. I myself have written my own memoir, and I’ll be 24 in a few days.

As for her behavior, it could be stress, it could be that she has an illness you’re unaware of, it could be anything. Approach her with your concerns, but not your accusations. Talk about how you’ve noticed she’s lost weight, she’s been under stress, be supportive.

Before you speak to her however, really take a look at your relationship. If there’s someone else who would better handle the situation, tap them for initial contact.