Привет, Наталия!
I don't understand Russian very well, but with a little help of GoogleTranslate (yes, it can be very misleading sometimes :) ) I understood that you're asking why has the author used the name Ruzhica. As you've said the name is made out of the name for a beautiful flower - the rose. The girl in this song says that if he doesn't love her: "Лицо мое увядает". And that's the explanation: she's a rose, and without her love, she'll be a wilted one. Her name is used for better and more symbolic interpretation of the grief and heart-ache the girl will feel if the loses the loved one, given with the image of a wilted rose.
If I misunderstood you comment, please do forgive me, and if not, I only wish I could be able to write this comment in Russian, I love the language. :D