Q. I met on Tisha Beav a friend who informed me of the recent engagement of his son. I told him Mazal Tov, but then I thought maybe one should not say that on Tisha Beav. What is correct?
A. Nitei Gavriel (Bein Hametzarim 2: 76: 7) quotes Leket Yosher that one may say good night on Tisha Beav since it is not a greeting but rather a blessing and a prayer, He mentions that the same would apply to saying Mazal Tov,
He proves his point from the ruling of the Ezer Mikoidesh (E.H. 21), that although one should abstain from greeting women, he may say Mazal Tov after a bris or other simcha, since it is a tefila and a blessing. He also quotes Shalmas Chaim (Y.D. 425) ruling that one may wish Mazal Tov to an ovel.
Horav Shlomo Miller’s Shlit’a opinion is also that one may wish mazal tov.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a