Q. Some people in my shul say that you can greet someone on Tisha Beav after chatzos (midday), when they come to daven mincha. Is that correct?
A. The prohibition of greeting on Tisha Beav applies the complete day. (Piskei Teshuvos 554: n. 86).
Nitei Gavriel (Bein Hametzarim 2: 76: 2) permits after midday in need.
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a