You don't, currently. Not in the way you're thinking.
There's no way, currently, to connect her Mac directly to your MC library, nor any way to run MC on her Mac (without installing Windows in a Virtual Machine).
That's not going to last though. For now?
Well, you can share the files directly off of her Mac under System Preferences > Sharing (see
How to Setup Windows File Sharing on a Mac). Then, you could import the files on her Mac directly into MC. Then you can play the files using MC on your PC any time her Mac is there, awake, and on the network.
Doing the reverse (importing files on your Windows PC into iTunes on her Mac) is more troublesome, but also not impossible. The page I linked above has instructions for that as well.
Be careful with this second item though. This is going to let iTunes on her Mac directly access your files. It can (and will) write tag changes to the files, and (assuming your share is read/write) if she deletes a file, it is gone from your machine as well (and won't go to the Trash Can).