If a person created a Windows 10 installer flash drive and use this flash drive to boot the Mac, then the person would have access to the Windows user files. The ISO file needed to create this flash drive is made available free of charge by Microsoft. The file can be downloaded from this Microsoft website.
For example, when booting from the October 2018 Windows 10 update, you will first get the screen shown below.
If you press the key combination shift+F10, then you will get a the Command Prompt windows shown below.
If a user account was named
dma, the one could change the directory to this users Desktop by entering the command given below.
cd /d c:\users\dma\desktop
Some newer Macs will have Windows installed with a Windows Recovery Environment (WRE). However, if someone tries to use WRE to open a Command Prompt Window, then they will be prompted to select a existing user account and will be required to enter a password.