There is a stunning arrangement of the British folksong "How cold the wind doth blow" by Vaughan Williams for voice and violin. The text is female (although with a slight modification a man can sing it) and the range is up to F, so OK for light mezzos.
I had to get a copy through ILL, but maybe CVR has it.
I'm sure there are numerous Bach arias that would work for this combination.
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