You'd have to use your App Store account for her phone. To be honest I see little reason not to, you end up sharing apps and not having to buy them twice!
Settings, iTunes & App Stores and you can sign in there.
That's how I do it in my household. We all have our own iCloud accounts but we all use the same iTunes account, that way any purchases can be used on all devices with no additional cost.
Why bother?If you want to use separate accounts then just temporarily sign in using the "other" account then you can download and install the app. Once this is done you can sign back into the original iTunes account and the app will still work, you'll just need to do the same thing again to update it in the future.
Why bother?
Yes you can do it, there's no way Apple can tell if you just have two iPhones or not. You can even have her log back into her own App Store account after and she'll keep your apps. She'll need your login to update them though.
That's why you sign into your own individual iCloud account. You can be logged into iCloud with one account and iTunes/Appstore with another (shared) account. It works perfectly and we have four devices in this house all doing that.I can see one reason, keeping iMessage separate. I share an account and sometimes get the other persons messages and vice versa when we fluff up the settings.