No, you will need to borrow someone else's DSi in order to update the Acekard now. You can't undo the DSi update. He really should NOT have done that update to your DSi.
I had the same issue when I bought one for my g/f's DSi - hers was already updated when she bought it, and the card Acekard sent me hadn't been flashed to the newest firmware yet. I just used my nephew's older DS to update the Acekard firmware.
Your only other option is to use a DS Lite to perform the update. I could be wrong but I don't know of any other way. I spent a few minutes googling around to see if I could find anything, but I think you're out of luck.