Yesterday I said "shabbat shalom" to someone and he said we don't do that on Tisha b'Av that falls on Shabbat. I thought that Shabbat trumps the day (and that's why we move the other observances). Neither of us knew a source, and it hasn't come up yet in 9 be-Av on Shabbos .
The Shulchan Aruch (i will try to find the exact place) says that on a shabbos thats also tisha b'av you should mour n for the bais hamikdash b'tzinah (privately) verbally not saying good shabbos/shabbat shalom is not exactly private and he was doing it because the day was tisha b'av. Although he might have a special minhag (custom) to not say good shabbos/shabbat shalom, you might want to ask your friend about that.