If I understand you correctly, regular cell calls made with wifi on have reception issues. Turning wifi off seems to resolve those issues. Is that right? (I was confused by what you said about calls "over wifi", since even with wifi on, regular cell calls use a different antenna.)
Nevertheless, this sounds like an unusual problem. It sounds theoretically possible that you could be getting some kind of wifi interference with your voice calls -- though the phones are designed to prevent this kind of interference. I have trouble with wifi while using a Bluetooth headset, so I will frequently disable wifi while using my Bluetooth headset to ensure optimum sound quality.
I've never experienced this problem while making calls using the phone's earpiece, however. Is Bluetooth on? Another possibility is if she is just on the edge of the wifi signal, there could be more interference.
Try this: does making calls with wifi on cause problems while on other networks? If problems appear only on the home network, do they persist even if she changes her physical location (eg nearer and farther from the router)?