Tina,
if she really is a mezzo of sorts, she should work on strengthening her head voice down there. I don't think it is good to rely on your chest voice past middle C. (I'm talking about for mezzos here) Actually I never use it above about an A or Bb below middle C. ANYWAY, I think the best thing to think about is keeping the same upward focus as in her upper voice. Don't let the sound get all fuzzy and sloppy... lower voice takes just as much concentration and breath as does the rest of the voice. If I just concentrate on making a bony, forward focused sound down there while retaining the resonance in the head and keeping the soft palate lifted (lots of things to think about), it will match the rest of the voice. Although, if she really is a soprano, now that you have worked out the kinks on the upper voice, she shouldn't mess herself up again by singing rep that is too low for her.
Lauren
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