If you go with the 1x7, she will be used to the sound and feel of the shift if she wants to continue, and get a new bike with more gears. You can also change the freewheel\cassette if she wants a closer or wider range of gearing.
You are correct sticking with a single cog up front, theying to learn to shift both when you are starting with a multi gear bike can be a noisy, frustrating challenge.
And some people, such as my wife, the "Queen of Cross Chaining" never really get it.