Electra Shifter (Revoshift) is hard to turn
I recommend getting a rear Shimano trigger shifter.  This would mean getting both a trigger shifter and a grip because I think your current grip is designed for the Revoshift shifter.

The part I'd recommend for this use is the shimano acera 7-speed trigger shifter, which costs about $15, if you're in the US.

If you are mechanically inclined, it's pretty easy to do the work yourself, and it's a lot of fun when you get it working.

If you're not mechanically inclined, bring it to your LBS to install.  This will save you the effort, and you eliminate the chance of irritating your wife for breaking her bike.