Take off all the keycaps. Disassemble the keyboard (take the PCB + plate out of the case). Spray some high content alcohol (the higher the better, at least 91%) all over to get all the visible coffee out. Dry really thoroughly (wait a long time, I'd wait up to a week). Plug in the cable, see if it works. If it doesn't work well, try spraying more alcohol actually into the switches or even good quality contact cleaner.