A DLC Unlocker works by intercepting these queries or altering the game's memory to "trick" the client into believing the licenses are present. Effectively, the game client acts as if the player owns all available content, despite the Steam or Epic Games Store account not having made the purchase.
acidicoala/ScreamAPI: Legit DLC Unlocker for Epic ... - GitHub
: Third-party scripts and executables from unverified sources often trigger antivirus warnings and may contain malware or trojans.
If your friend owns the DLC on Steam:
You are using content created by developers without compensating them, which can impact future game support.