MacOS – Skip an OSX version with App Store upgrades

An install on Mac OS is generally just copying the relevant files into place. Usually, you'll be able to copy an app's bundle over from one machine to the other. The only thing is that the App Store will have some sort of DRM, so just copying it may not work. I suspect that the answer is no, because you'll need to sign in to the App Store, but I haven't tried it. (On the other hand, I've installed Lion on a Leopard machine via a disk without Sow Leopard.) Additionally, Apps that are downloaded from the Mac App store will carry minimum system requirements that might prevent them from running altogether, even if you theoretically circumvented the DRM from the App Store.

Yes, although there are many questions that cover the same ground already on SE.

You can DL app store apps (including Lion) on your girlfriend's computer. Then she can log into her own apple account and buy other apps. The only thing you need to be aware of is that for updates, you can only update the apps you bought with that particular account. So if you want to update Lion (not likely as it will come through software update) or other apps, you will have to re-log into your account.

I believe most apps have a 5 computer limit.