In theory, you might be able to copy paste the baldur's gate files from your steam library directly to her library folder. When she tries to install BG3 on her Steam account, Steam 'should' find the files mostly already there, and verify that you have the game files, and then it'll do the rest, such as registry keys, etc.
That said, I would just play it safe, and do it the regular way. The game isn't going anywhere.
Last edited by Boblawblah; 02/08/23 05:52 PM.