This correspondent is identified in the published Igros Kodesh only as "Shifra Yehudit" (about 5726–5730, 1966–1970). The letters appear to be to one woman, active in supporting widows and in communal work in Eretz Yisrael.
Supporting widows. The Rebbe encouraged widows who lead support efforts for others like themselves (IK 9414), and encouraged steadfast faith in difficult times, highlighting the unique role such women play (IK 10252). Encouragement and comfort are best delivered personally (IK 10097).
Community guidance. He guided her on proper bar and bas mitzvah celebrations under the circumstances of the time (IK 9516), and advised keeping to accepted community standards on women driving, for reasons including safety (IK 9872).
Sources: the Rebbe's letters cited above. See also Mourning and loss – Widows and orphans.