Hi, I'd like to help you with your question.
No, this does not necessarily mean she has low self esteem. People often take on the affects of others, particularly if they like them a lot. It also shows affection for the other person and a desire, conscious or unconscious, to be close to that person. It could be that your daughter is a particularly affectionate person and likes her friends a lot.
If you feel that your daughter may have low self esteem or identity issues, look for other symptoms in her life. She may talk bad about herself, put others opinions before her own, taking responsibility for others' mistakes, be unwilling to stand up for herself, only feeling ok if others accept her and has difficulty expressing her own opinion about anything.
If you find your daughter has self esteem issues and she wants to work on them, she can see a counselor. She probably would only need short term therapy, just to help her explore where these feelings came from and to learn new ways to feel better about herself.
A book she can try is called The Courage to Be Yourself: A Woman's Guide to Emotional Strength and Self-Esteem by Sue Patton Thoele. It can be found at Amazon.com or your local library may have it as well.
I hope this helped you,