You should end your friendship when there is an issue of trust between both of you and you think you aren't safe with them.
This friend may have betrayed you more than once but you choose to forgive, but when you can't take the gossiping, back biting and bad blood anymore, it's better you let go for your own peace of mind.
If it hard to avoid this person, it's best to turn them into an acquaintance. Be friendly but never reveal your deepest secrets to them. Just be polite and nice instead of warm and open like it used to be.
Trust is the basis of every friendship and like a foundation, once it's broken, there's little or nothing to stand on.
Watch this short video by Marie Dubuque explaining when exactly is the right time to end a friendship.