Not from opera, but very high quality: Domenico Argento's "Letters From the Composers". Written for guitar and soprano. Based on actual letters by Bach, Schumann, et al. Contemporary tonal music; needs a soprano who can do lieder and opera. Your soprano will likely love these, and they're great fun too.
Also remember the excellent transcription of Bachianas Brasilieras of Villa Lobos for guitar and soprano. A chestnut, yes, but she needs to be good. (Parkening recorded this with Kathleen Battle.)
Something lighter, but gorgeous: Four French Folk Songs, Mathyas Seiber, guitar and high voice. May be hard to find, but absolutely gorgeous pieces, haunting melodies, one is quite amusing too.
2015 Connor spruce/Indian rosewood
1978 Ramirez 1a cedar