She can install any apps you purchased (or vice versa) without repurchasing. The trick is to add the purchasing account info to the store in the other account.
So if you buy a game from your account, under your girlfriends account, in the store settings go to "Your account" and hit the "change user" button. Add your account info. At this point even under her user account, the store will be using your account and when she goes to "Your apps" (right click or swipe from top in the store) she'll see the game you purchased and can select it to install. Installing it will just essentially set the tile up on her start screen; it won't need to redownload if it's already installed under your account.
This will work whether she has a local account or Microsoft account. If you're wondering, game saves/progress/achievements will be under the user account of the player regardless of who purchased the game. So her saves, etc. will be separate from yours.