You can definitely prune your devices list in iCloud's "Find iPhone" (as it's called on the iPhone itself) but here's the trick: You can't actually update devices on the devices themselves. Just a quirk of how the system was designed, you need to log in to the iCloud service on your computer with a regular Web browser, not from within the app.

If you're using iCloud for data synchronization, it's worth checking out how nice a Web-based interface Apple's created for the service too, however, so this is a good five minute exercise anyway. :-)

One thing you'll quickly realize is that you can get to your email, calendar, contact list, and a lot more, all without having your iPhone handy. Might help out your wife. (I didn't say that!)

Log in to iCloud with your usual credentials by going to iCloud.com. You'll see this:

You can see I have a pending reminder. It even lets you interact with those, and especially notice the "iWork" area too. All ready to roll. Easy.

We're going to stay focused, however. Go to "Find My iPhone" and you'll see a map with all known devices pinpointed:

In my case, I have my iPhone and my MacBook Pro computer, so there are two green dots clustered together at the coffee shop I'm working at this morning.

Notice on the top left the button "Devices". Click on it.

Look closely and you'll see that each and every device was "Located less than a minute ago", which means that they're all powered on and able to access the Internet, regardless of where they are. That's exactly the situation we hope for, so finding my iPad Mini, for example, is as easy as clicking on its name on the list.

If I scroll down a bit, however, there's an obsolete device on the list too:

I sold my old MacBook Pro (MBP) and while the new owner has reinstalled the OS, a smart move, I forgot to remove it from Find My iPhone, so it's exactly the situation you're seeing too. To remove it, I click on it on the Devices list, which shows an "Offline" error:

Look closely at this screen and the link needed proves to be at the bottom:

Click on "Remove from Find My iPhone" and it'll tell you what's about to happen:

Good with that? Click on "Remove". Now that'll vanish from the Devices list on every device that's logged in to your iCloud account and accessible for Find My iPhone.

And now ya know!