The way I have always played is that you know the base result of both rolls before you decide to spend any fate points or make any aspect invocations. Then you can invoke aspects, spend fate points and tell the story of the interaction. You may even get into fate point "bidding wars."
Under that interpretation there is no difference between spending fate points before or after you declare a Riposte (and we aren't going to punish people for doing things technically out of order). In fact, I would let the player use aspects like Cover to increase his defense (and wouldn't be useful on offense), and then also let him bring out aspects like "Pull No Punches" (which wouldn't be useful on offense) to increase the damage of the Riposte attack.
Basically he would describe the action as "I had taken cover behind the bookshelf. Then the Goon turned the corner and swung his sword at me, but he was surprised that I had stopped and hadn't kept running, so his attack was off balance (Invoking the Cover aspect to increase defense). I took advantage of his stumble to deliver a Riposte, and I really let him have it, I don't pull and punches (spending fate point to invoke his own aspect)."