Thanks for the info. Here are my comments - other people might have different views

Unfortunately the customer's answers (a) and (b) are too limited to get useful meaning - no Windows error messages which they saw, or Windows commands which they tried, or details of what they did physically. So that does not help with diagnosis.

Based on your answer (d) I'm surprised at answer (c) - why didn't you try to clone it? Was there a problem doing that, which you didn't explain yet?
Since you didn't mention that you had tried cloning the drive... After checking for any obvious physical damage then, based on the limited dmesg output, your earlier comments, and your lack of a specialised NAND reader, one of the few things you can try is making a raw clone of /dev/sdb to a suitable target device or image file. Personally I would use GNU ddrescue, with the logfile writing to another working filesystem, and of course the direction of cloning (source -> target)
must be correct! If any filesystems from the USB drive were mounted by your Linux distro during booting, they should be unmounted first.
As this is a customer's drive, I wouldn't try those tools mentioned by
HaQue due to the risks which he mentioned.
Next steps depend on the result of the cloning attempt - or, of course, you could outsource this job to another company with more experience with these devices. If any of the raw clone process is successful, that data needs to be checked to make sure it is sensible and not (for example) all 0x00 or 0xFF. Good luck!