Frankly, you don’t. It is almost impossible to take apart the case intact and have it still be working, hence the 0/10 repairability on the repairability scale. I would just advise your customer that it might be worth their while to buy a new charging case. This is a rare case that I advise doing something like this but it may be your only option.
Point your customer to this website and have them contact Apple for a genuine replacement charging case.
https://support.apple.com/en-us/HT207827