You can chisel off the concrete with a mallot, and you don't have to get it all off. You can leave about a 2" radius attached to the swingset. (If less ends up happening, no problem.) The point is, that the 'residual' concrete on the posts will 'grab' the NEW concrete you friend uses to set the ladder in HIS yard. If you try too hard to get all the concrete off, you may end up inadvertently ripping some of the wood off of the ladder.