Water Damage Long term problems

My cousin dropped her iphone 4 into a sink last october, at the time she took it out of the water and dried it as much as possible without actually taking it apart.
I didnt want to open the phone because she only got it a month previously and i was worried about warranties etc, but now i realised the phone has no warranty on it as such as it was second hand but in new condition.

The symptoms
- Keeps locking constantly ie turning the screen off like the power button has been pressed even when it hasn't
- flash does not work
- power button intermittently doesnt work at all
- the screen flashes when the phone locks occasionally

Would there be a specific part of the phone that would be damaged, is it repairable?

Water damage is unpredictable. Since it's been awhile the phone was dropped into water it's very likely that the interior has been damaged by corrosion. If you feel adventurous, take it apart and examine it to see if perhaps it can be repaired.

The take apart instructions can be found at iFixit: The free repair manual.

The problem with water damage is that it's hard to say what the outcome is going to be. If the physical switch isn't damaged, the water would definitely damage the logic board causing all sorts of random behavior as you are seeing.

The specific part that is most likely damaged is the major part, i.e., the logic board. It is also possible that other circuitry, like the flash and so on are damaged as well..

Apple introduced a accidental coverage to cover damage to iPhones and with a small deductible they would replace a sufficiently broken iPhone..I don't know if that's available where you are and if your cousin purchased it..

By the time you go around replacing all the parts that could potentially be damaged, you might as well take it back to Apple and get a new one paying half the price or what have you..

Alas all toom often water + electronics = death.