since you are virtually screwed as far as repairs, i'd go to an ifixit guide and open the whole thing up, take the top case off, harddrive out, leave the battery out etc.
The top case isn't too hard to get off, so i would start with that and then get some isopropyl alcohol (highest percent) and a cue tip and do some gentle cleaning becuase like people said, sugar is bad bad bad bad bad
Clean it, use a dryer (ON COOL SETTING) and blow on it a bit and then let it sit open upside down for another day or so, and put it back together and try it again.
You may want to pop the keys off and clean that too as they might get sticky if they aren't already.
be VERY gentle cleaning the contacts with alcohol, but do a thorough job.
Perhaps some compressed air as well as the hairdryer to push water out that might be hiding.
Try it again and see what happens a day or so after cleaning it out.
IF it works, tell your friend he is lucky, but if you have to take it in for repairs down the line, and they note water damage (colored contacts etc, alert this sometimes) and refuse to fix it for THAT reason, he should pay.
If it doesn't work... he should pay for a replacement - or the difference anyway. You can probably sell a water damaged macbook on ebay for 400+ depending... so he should pay for the replacement and then you can give him the water damaged one to sell. (minus your harddrive)