If your MacBook supports AirPlay, you can easily use a Windows laptop as a second monitor by following these steps:
- Connect the Windows laptop and MacBook to the same Wi-Fi network.
- Download and install AirPlay software on your Windows laptop.
- Enable AirPlay on MacBook.
- Launch AirPlay on Windows.
- Extend the Mac display to Windows Screen.
On the other hand, if you are using an older MacBook that does not support AirPlay, you can use a tool like AirDroid Cast to screencast your MacBook to a Windows laptop.