Kadabra didn't learn confusion since Abra was evolved a level too late.
Upon reaching level 16, Abra should've evolved, and Kadabra would've learned Confusion, since Kadabra learns Confusion at level 16. However, since Abra reached level 16 and fainted, its evolution was delayed until level 17, meaning Kadabra missed out on learning Confusion which it would've learned if it had evolved at level 16.
Kadabra does learn Psybeam at level 21 if you're willing to wait a bit longer. I'm assuming you aren't too far into the game, so by the time you reach the move reminder and have a Heart Scale, you should have much better moves and Confusion won't be worth remembering.
Alternatively, if you happen to have a damaging move TM and don't mind the fact that TMs are 1 use in Emerald, you can use a TM, although I don't remember if there are much TMs Kadabra can learn so early on.
Edit: Abra and Kadabra both learn Thief by TM which you can get at the museum in Slateport by talking to a Team Aqua grunt on the first floor. Thanks to sumwun for mentioning Thief.
If you're truly desperate and don't mind losing progress, if you haven't saved after Abra evolved, you can always reload back to your previous save and try evolving Abra again at level 16, although I would not recommend doing this.
Hopefully, everything I said made sense and you are able to find a work around.
Source: Experience.
