Let’s say a friend was quite clumsy and spilled a glass of water on a laptop when the laptop was turned off. What are the effects of that likely to be?
I hesit - I mean, my friends, hesitates to turn it back on. Is there a certain number of hours my friend should wait?
More generally, how fucked is my friend?
Open it and flip it over so gravity is on your side. Take the battery out NOW. Anything else you can take out, take out. Stick it somewhere dry with good airflow. In front of a fan would be good.
If the water was clean and hasn’t been touching powered-up electronics, once it has all dried, there’s a chance you’ll have gotten away with it.
I would personally wait 48 hours or so to turn it back on. Most of the water should dry out of it overnight, but you’ve still got a chance of a tiny bit of water remaining and that could cause you some problems.
Most electronics are relatively waterproof if they weren’t powered up at the time, so there’s a decent chance your friend’s laptop could come out of this ok. There are some circuits that are still powered even when the laptop is “off” though (otherwise the power button wouldn’t work) so there is a chance of damage.
One of the Geeklings spilled water into Mrs. Geek’s laptop while it was powered on. She turned it off quickly and I immediately took the battery out but she ended up with a bad keyboard out of all of it. Keyboards are reasonably easy to replace in a laptop so once I put a new keyboard in everything was fine. So even if the laptop had been turned on at the time it would still have halfway decent odds of survival. Your friend’s odds are a bit better since the laptop was off at the time. There’s always a chance of damage being done, but there’s fair chance the laptop will come out ok in the end.
If you are in a particularly humid area (like South Florida) then putting the laptop in a box with a desiccant like bags of uncooked rice will help draw the moisture out of the laptop. If you are in a place with normal humidity levels then that won’t really help you in any meaningful way. In the latter case, just letting it sit somewhere for a good long time is the best you can do.
DO NOT blow on the laptop with a hair dryer. Hair dryers produce static electricity that could damage the laptop.
ETA: matt’s advice is important enough that it’s worth repeating. Take the battery out NOW if you haven’t already done so.
First, the laptop is NEVER fully powered off. The battery is powering the unit at an ultra-low level. The power switch only tells the CPU to light up the motherboard and the rest of the processor.
THAT is why Matt said to yank the battery.
That said, water IS highly corrosive and most waters (other than distilled) are rather impure and will tend to leave a residue that can potentially cause problems.
Personally, I’ve used high percentage alcohol, liberally applied to the unit.
Alcohol is hydrophilic, which means it really, really, REALLY likes water. It’ll mix quickly and easily and also help evaporate the water (there is a boatload of annoying chemistry behind that one).
There are other chemicals that would work well, but are more hazardous, either due to flammability or toxicity or both, such as certain ethers. I’ve used two different ethers, but only under a hood, as I’m really big on staying alive. 
So, either dry the thing at warm temperatures for a week or two OR dissect it and use the alcohol.
NOT the rubbing alcohol kind that you get at the drug store, go for the 90%+ type. Either the 90% alcohol at the drug store or use my favorite, 190 everclear, available at your local booze shop. It makes a horrible drink, undiluted, but one of the better solvents for many, many things.
I’ve even used it to extract vanilla in a hurry, then watered it down for use. But, that is ANOTHER story.