MBPr 15" won't turn on

My friends MBP Retina Mid 2012 just won't turn on. She went away for 4 days and left her computer unplugged and asleep at home. When she came home it worked fine, until it suddenly turned off at 50% battery. Has been unresponsive ever since.

When you press the power button, nothing happens. The light on the charger doesn't even turn on.

I have the same computer, so we've ruled out all obvious causes.

What we've tried:

- Holding down the power button for 10 seconds

- SMC reset using the Shift-Control-Option key combo

- SMC reset by opening up the lid, disconnecting the battery, etc

Note: I don't know if you can actually call it an SMC reset, because the computer is just unresponsive.

- Tried her charger on my MBPr, works

- Tried my charger on my MBPr, works

- Tried my charger on her MBPr, doesn't light up

- Tried my charger on her MBPr right after disconnecting it from mine, lights up to orange or green, depending on what it was when I disconnected it from mine. But the weird thing here is, that it fades out really slowly until it eventually turns off completely.

We managed to recover all her data by swapping the SSD's and booting it up in my MBPr (she didn't have a backup). During that time (about 4 hours), I left the battery on the faulty MBPr disconnected.

She should probably take it to Apple and I think she's learned her lesson on the backup part, but I'm just curious if you guys can come up with some other stuff we can try. Thanks in advance!