I can't really answer since it would require proper testing and I dont know how steam handles multiple accounts, but i can help you play safe meanwhile:

When you launch skyrim, in his launcher, you can disable the DLC by pressing the "DATA FILES" button Now you should be able to see a menu with various files ( mods/addons and DLC'S).

If you have the dlc and he recognizes them, you can disable them by clicking on the checkbox ( for example Dawnguard.esm is gonna disable the dawnguard DLC ).

Then press ok and play the game without the DLC's.

For safety, i recommend to make a backup of her data files, copying them somewhere safe before playing Backup save files

I can't really answer since it would require proper testing and I dont know how steam handles multiple accounts, but i can help you play safe meanwhile:

When you launch skyrim, in his launcher, you can disable the DLC by pressing the "DATA FILES" button Now you should be able to see a menu with various files ( mods/addons and DLC'S).

If you have the dlc and he recognizes them, you can disable them by clicking on the checkbox ( for example Dawnguard.esm is gonna disable the dawnguard DLC ).

Then press ok and play the game without the DLC's.

For safety, i recommend to make a backup of her data files, copying them somewhere safe before playing Backup save files