No charging light, won't charge, goes on when battery is unplugged

Hello everyone, I’m not actually an avid Mac user, just trying to figure something out for a friend and it’s my first MacBook I ever even opened. So here’s the deal:

I plugged the charger, there’s no light, laptop won’t boot but there’s a battery sign on the screen warning the battery level is very low - it’s red and blinking, like every 10 seconds it shows up then disappears and so on.

A friend of mine informed me, he checked with other chargers, still the same problem. Also, he said there’s been some wine spilled on the keyboard, although it’s worked fine for couple of months after this accident, so I don’t know if that’s the issue.

Anyway, I opened the laptop and actually it didn’t look so bad. I checked battery pins with multimeter and it’s practically empty, fully discharged (0V / ~100mVs) so I decided to disconnect battery and check if it would help. And suddenly, when I plugged in a charger, but while battery was disconnected from the board, laptop went on - without me even touching a power button, it just turned on.

So when battery is plugged, it’s dead - no light, just low battery signal on the screen. When the battery is unplugged from a board, laptop goes on by itself when is connected to the charger, with no light obviously (yeah, even if laptop is on there is no orange light on the charger). Like I don’t have to touch a power button, it just turns on when plugged in.

So my question is, is it possible that battery/battery cable is dead, or maybe this might be a short somewhere on the board and where would you start seeking the problem? I thought maybe I should remove a motherboard and check if there is any liquid damage on the other side which I haven’t seen yet.

I also read somewhere it may be keyboard related, that’s another option. Eventually, I could try to put 8.6V / ~2A from external power supply on the battery connector and check if battery would take any amps. I don’t know,

I’d like to ask you guys if you have any ideas or patterns you use while testing motherboards.

Any help is much appreciated and I'm looking forward to your replies.