Most Helpful Girl
You can't influence how a girl feels about you or what she'll tell her friend. Girl talk is just girl talk. You can let your interest in the girl you want known, but asking her friend to put in a good word for you comes off lame and slightly manipulative. If her friend wants to put in a good word then she'll do it without you asking. She won't do it because you prompted her to do so.
Now if the girl is YOUR friend and not really hers, then she might do that.