Spilled water – flashes dark overlay on segment of screen

More than likely liquid either got onto the back of the board, or into some bit of the display. The spot where water got in is RIGHT near all the backlight components on the board. The connector to the right of the fan, near the top of the board is for backlight power. So it would make sense that on the other end of that cable somewhere is some backlight circuitry for the display itself. Which could cause this one strip of the display to be dimmer than the rest/flicker. This could be a you need a new display sort of deal, or it could be something on the board, not sure if your savvy friend took the whole board out to check for any damage or if they just opened it.

I'm leaning more towards display being the issue, but these machines are new enough that I haven't gotten to actually do much with them other than set them up for use. And tear one down to check for liquid damage (Apple ended up fixing that one under warranty through some miracle).

If you don't immediately power down and thoroughly dry it out the current flow causes corrosion. Looking at your pictures I THINK I see some blue - can't tell for sure - it isn't sharp enough but I would open it up, power it down (disconnect the battery too) examine the board closely for any corrosion and clean it off very carefully if found. If you aren't comfortable with thatpernaps your friend can help. If you find any you really ought to check the back of theboard and the screen connector too, if you find parts that are damaged you might need to have to get professional assistance if your friend is good at his job he should be able to guide or assist you. I'm nowherebut suspect there's a guide on tearing it down. Good luck if you do find corrosion try alcohol and a soft small toothbrush on it... dry thoroughly before reconnecting the battery and trying it out. With luck cleaning the corrosion will do the trick.