Hello Elvis,
Did your friend completely delete the Mojave partition? Is there only Windows 10 on your MacBook Air? If s/he used Bootcamp to create the Windows 10 partition then your Mojave partition should still be there unless it was deliberately deleted to ensure that you only use Windows on your MacBook Air. As your friend what was actually done.
Meanwhile check to see whether you still have Mojave partition to boot from -
- Restart your Mac from the Start menu in Windows.
- Press and hold the Option (or Alt) ⌥ key as your Mac begins to restart.
- Select your Mac startup volume (Macintosh HD) in the Startup Manager window, then press Return or click the up arrow:
If you were able to boot into Mojave then your Mac partition should be just like the way it was before Bootcamp was used. If you were not then you can confirm that the Mojave partition does not exist.
If the Mojave does not exist then have your friend come over and use the ➔ macOS Recovery Mode to reinstall Mojave back on your computer. You friend can also use DISK UTILITY, while in Recovery Mode, to delete the Windows partition.
Axel F.