It seems to me you've failed to correctly identify the issue. It's not .JPG vs .JPEG. It's that iPhones now capture photos in a high-efficiency format by default; your client prefers the JPEG format; and you haven't gone into the Settings on your iPhone to tell it to use JPEG.
I suggest that you change your Settings, following the instructions in:
Using HEIF or HEVC media on Apple devices - Apple Support
"Though capturing in HEIF and HEVC format is recommended, you can set these devices to capture media using the older formats, which are more broadly compatible with other devices and operating systems"