On the 7D you have a 'green dot' focus light in the bottom-right of the viewfinder. Regardless of if the lens is in AF or MF mode, this will light up when the image is in focus (or at least, the part under the active AF point in the viewfinder).
I use this a lot as I have the 7D, and a Zeiss 100mm makro-planar lens which is ONLY manual focus (a gorgeous, gorgeous lens), and the difference between hitting that plane of focus and having it a millimetre out is next to nothing! So the green dot really does help.
The beep on AF will still sound on the 7D as soon as the green dot comes on. This may happen several times rapidly in succession if you're 'hovering' around the right focus point. If it beeps and then you (or she moves) then of course it will be out of focus again.
If I may make a suggestion: Go into the custom function button config and change the shutter button to take the picture only. ie. so it does not trigger AF or metering. Then make sure the rear AF-ON button is set to do the AF and metering. This way, you and your g/f can just keep your thumb on the AF button whilst looking through the viewfinder and as soon as the green dot comes on you press the shutter and you have your picture!
To answer your original question though - no - you cannot get the camera to 'warn' you when it's not in AF mode. However if you look on the right of the top display, where it shows the options for ONE SHOT, AI FOCUS, and AI SERVO modes, if the lens is in MF mode, those won't be shown at all. As soon as you flick it back into AF mode the currently selected AF mode will be shown. So you could always check that?