It sounds like you're trying to address the symptom, rather than the problem. To be clear, Messages should not be using up that much battery power. I suggest the following:
- Restart your wife's iPhone by holding down the Power and Home buttons simultaneously for about 10 seconds until you see the Apple logo
- Wait for her iPhone battery stats to refresh.
This may take up to a day - typically it will need to be plugged back into a charger and fully charged and then removed and used normally for a few hours up to a day before the battery stats will refresh.
Once you've done this, let us know if your wife is still having this problem.