"Wrong passcode" after battery replacement

The Force Touch Sensor is the only form of waterproofing that an Apple Watch has, making it important to replace during a repair. However, it should not be causing the type of error that you describe. That type of error is probably caused by the charging troubles or some kind of error within WatchOS itself.

Here is a support guide from Apple that should help resolve that issue: Link. It should help to have the owner of the watch use their iCloud-linked devices to get through that.

Here is a link where you can buy the Force Touch Sensor: Link.