Read up about Depth of Field and hyperfocal distance. It might be possible to get everything acceptably sharp as long as the subject isn't too close.
Or, if you want to achieve what optics won't allow, take a shot of your wife in position and then another with her out of the way (using a tripod or other support so the camera doesn't move between the two). You should then be able to blend the two pictures together.
The best option of all might be to take some wonderful shots of the falls and some where your wife is clearly the subject and just put them side by side in your album.
Wulf