Minecraft Windows 10 Edition will only runs Windows 10.
If you want to play Minecraft on a non-Windows 10 PC (i.e. Windows 7, Windows 8, Mac or Linux) then you need the Minecraft PC/Java version. The two versions are not compatible with each other, so you can't share the same world save between the two. Yes, you will have to buy the PC/Java version separately.
According to the APK website: "With APKPure, you can instant download region locked (not available in your country), pre-registered games and many other apps on Android devices." And Bluestacks is just a program that lets you play Android games on your PC. Therefore, you could purchase the Pocket Edition (an Android version of Minecraft) to play on your Windows 7 PC, but not Win10.
The Good News is the Pocket Edition is basically the same thing as Win10 and they can play together (currently they're the only two editions that can play with each other).
The Bad News is you'd still have to purchase the Pocket Edition separately.
In other words, you can get there, in a round-about way, but it's gonna cost ya a bit. Hopefully it will work.
To me, the best option (but most expensive) is to get her a new laptop with Windows 10 on it. Not only Minecraft, but all her programs/games will appreciate it. (If she's running Windows 7, it's old and about time for a new one). Of course, another option is to buy yourself a new laptop ( ) and give yours to her!
Another even cheaper option is to:
* Get her a Windows 10 tablet. She can play the Win10 version of Minecraft.
* Get her a Android tablet. She can play the Pocket Edition of Minecraft. Remember that you can still play together that way.