The Henna for Hair forum includes posts about rose water for fragrance (based on my limited
search). Same thing at Henna Sooq. On the Long Hair Community Discussion Boards someone swore that rose water abolished the scent of the henna but made no claims for interaction with or enhancement to henna. But I would think the clerk is correct about it helping to release the dye somewhat, since it has a pH balance of about 4.1-4.4 (about the same as wine, but considerably less than lemon/lemon juice and significantly more than amla).
Happy New Year to you too!!