my friend made a statement to which I replied " After 2 weeks you won't have the same say " now I don't know if that sentence is correct I think " after 2 weeks you will not have the same opinion " is more correct
The latter is correct. You can rephrase your first statement to be correct by saying "After two weeks, you won't feel the same way." or as you said "After 2 weeks, you will not have the same opinion."
Edit: I see what you were going for now in the first sentence. You can say instead, "After two weeks, you won't have the same opinion.".. "The same say" doesn't make sense.