Has she sung in the choir before? Is your choir director trained, or a volunteer amateur? The job of the choir director is to direct, and the job of a church choir director is to do it as diplomatically and kindly but firmly as possible- and that includes blending. I'm not currently a choir director, but I have done so in the past, and I just smile sweetly at everyone, remind them to sing so they can hear their neighbors, do a vowel-matching exercise or two, and rave about how wonderful they sound when they make the attempt. And of course pass on any compliments from the congregation about how great they sounded!