Well, at first I really thought the slap was uncalled for BUT in retrospect, here's why I now think it is justified.
Here's a man (milsi) who is seeing his friend (prachi) being treated disrespectfully by her fiance on many an occasion. First time, u watch from the sidelines. 2nd time, u decide such behaviour is unfair and unwarranted considering the kind of girl prachi is. Third time, u decide to take the matter in your own hands. (that is, not counting the no. of times neev and prachi have "fought/argued")
So, there you go, from Milind's point of view he's just standing up for a friend and defending her seeing how helpless she appears to be. Considering that he's also in love with his friend, his protective instincts and latent feelings for her are bound to cloud his judgement. either way, one can tell he acted purely on instinct and it surely was an act of impulsion.
having said that, the slap could have been made redundant.