Here is iFixit's product link (you may find it cheaper somewhere else): 2018 Macbook Pro Logic Board It appears to me that it is designed in such a way as to be part of the motherboard. HERE is a video of the tear down of your device. Also, here is another purchase option for the logic board but as it is ebay, you need to take precautions: USED Logic Board
To get the obvious out of the way first, make sure to power it off if it isn't already. Don't charge it either. If your mom still happens to have Apple Care on the phone, that is the route that needs to be taken next. However, if not, you need to get the phone apart and start cleaning with Isopropyl Alcohol 99%+ (available at electronics parts stores) to remove all traces of corrosion and water as soon as possible. Here is what @jayeff posted on another water damage question and I fully believe it is VERY relevant here: The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the phone's operating design and could damage the components. The corrosion starts immediately and is ongoing until it is completely cleaned away First do not try to charge or to turn on your phone and then remove the battery as soon as possible from the phone to minimize any further damage. Then you need to dis-assemble the rest of the phone and clean all the affected parts using...
