how do i get to know when to break friendship and get rid of that friend

Ignore him/her.Block that person from all the social site and from phone too

One of my friends used to hate another friend of mine. Later on, she told me that I should keep away from the other friend or she'll not talk to me again. I said to her that I love her and will always will but I am never going to trade a friend for another. I am still on good terms with her but we are not close anymore.

ignore, don't contact and pretend there is no such person....

Simply start keeping a distance and try to avoid her