This may or may not be relevant but keep in mind that you need to specify the action you're readying. You don't just ready "an action".
Also in my opinion, unless the condition is specified more than "when he casts a spell", I'd rule that the PC would need to use it or lose it after succeeding on the Spellcraft check against the Feather Fall.
Now if the condition was something like "casts and offensive spell" or something similar, I'd allow the action the be readied until an offensive spell is identified as being cast, or until the PC's next turn, when theyd decided if the want to act or try readying again.
Thirdly, the tactic the NPC wizard used in this case can be seen as either extremely canny or annoyingly metagamey depending on this GM's past playstyle. If his NPC's frequently can apparently useless spells to throw off ready counters, I'd expect this to happen even if a PC hadn't readied an action. If it only happened when a PC readied to interrupt a casting, I'd consider it metagaming and get frustrated.
I'd at least expect some sort of Sense motive check by the NPC to figure out my character's intent.