Two ways, depending on when the crash happened.
Right after the crash: the FiveM crash window has a Save crash information button. It saves a CfxCrashDump_….zip; have the player save it somewhere easy to find, like the desktop.
Any time later:
- Press Windows + R to open Run.
- Paste
%localappdata%\FiveM\FiveM.app\crashes and press Enter. - Sort by Date modified and pick the
CfxCrashDump_….zip from around the time of the crash.
If FiveM is installed somewhere else, the crashes folder is inside that install’s FiveM.app folder. Either way, have them send you the zip as it is; there’s no need to unzip it.