1)You told me you would spend this weekend with you friend.
2)You told me you would spend the last weekend with your friend.
Lets assume that he told it on Thursday and the weekend he meant was over. Which one would be correct? I guess it would mostly be the second one but I have a little doubt whether the second one would be applicable in any situation. Maybe we can use it if the weekend is not over yet. Am I right?
"This weekend" never refers to the past. It either refers to the upcoming weekend or the weekend you are having right now.
Sentence 2 is OK if you drop the "the." ..."spend last weekend..."