I'm going to kiss the friend of a girl who I had been dating.
Why are you going to betray the girl that you have been dating?
I love her but I know she doesn't love me.
You should find out what the girl that you have been talking to truly feels about you, before you ruin your connection. Also, coming between a friendship is wrong and will leave you hurt and alone in the end, especially if they have a great friendship.
Quite the contrary. I don't like her and she doesn't like me.
Then just break up the honest way. You don't need to cheat.