Huawei P9 lite battery issue

The problem is that milk is corrosive so it may have damaged the plating on the flex cable connectors. If the flex cables are damaged they can easily be replaced (once you get the case open and can get to them), but if the connectors that the flex cables connect to are damaged then that is a much more expensive fix as these connectors are not replaceable (or sold anywhere that I know of - I suspect that they are either made in-house by the people that manufactured the PCBs or they are made for them & not available on the secondary market). If it is the connectors then you will have to replace the boards that the connectors are mounted to and, depending on the number/type of boards (motherboard, fingerprint sensor, cameras, etc.), it may be cheaper to replace the phone.