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Was she using your account?
The only way you could access her save is if you both used the same account.

The mods are downloaded to your system and are again tied to the account used.

Originally posted by HikariLight:

Was she using your account?
The only way you could access her save is if you both used the same account.

The mods are downloaded to your system and are again tied to the account used.

OP's friends local saves and downloaded mod files were most likely left from the time they used the computer as most games save directory and mod folder are predefined and will be same if multiple accounts use the same computer.

Originally posted by chickabumpbump:

My friend played her save file for bg3 on my PC, she loaded her game and now her mods and her save files are also on mine.

How can I delete her stuff on my pc without it affecting on hers

As long as cloud sync was enabled and worked, your friend should have the saves in cloud and Steam should auto-install their subscribed mods (assuming they are from Steam Workshop) on any computer they install the game on. Just make sure you are on your own account and they did not leave their account signed in when they stopped playing. Then you should be safe to just delete the local files they left behind.

You may have to delete them while game is running though as Steam fetches cloud data when you run a game and syncs to cloud when you close a game.