I don't think that "etiquette", per se, is a real issue. If the previous author's result was published, it was no doubt considered to be a useful contribution, even with the proviso (conditional). You don't have to remind everyone that it's OK to publish such results.
Simply state the facts: "Smith [1] provided a proof of theorem X under the assumption that Y is true. Here, we present a new proof without the necessity of relying on theorem Y"