How did you encrypt the external hard drive on a Mac? If you use macOS built-in methods including FileVault, choosing an encrypted file system while formatting the drive in Disk Utility, or encrypting the disk via Finder, you can not access it on another OS.
You can use your friend's MacBook Air to disable the encryption on the external hard drive, it seems that the drive could be formatted with APFS or Mac OS Extended, an incompatible file system on Windows, you may need some third-party tool to read-write APFS/Mac OS Extended drive on anther OS after you decrypt it on macOS.
It is suggested to encrypt the external hard drive for macOS and Windows with iBoysoft BitLocker for Mac, it enables you to encrypt NTFS, exFAT, and FAT32 drives with BitLocker on Mac, so that you can safely use an encrypted drive on both platforms.
