1) Although it is forbidden to go to a person before praying to say “Shalom”, if someone else approaches a person and says “Shalom”, he may respond. It is even permitted for a person who has already prayed to deliberately approach someone who has not yet prayed and say “Shalom”, even though this will cause the one who has not yet prayed to respond with “Shalom”. (Mishne Berurah 89)

2) The prohibition to go before the prayers to say “Shalom” to someone is even if that person is his father or teacher. (ibid)