Buy her another mount instead. Mask mounting a video camera generally results in unwatchable video since every time she shakes/moves her head so does the image. Or she'll have to do a lot of editing later to produce something watchable. Or we all get to know every time she clears her mask during the dive. I shoot conventional housed video, sometimes when I forgot I later see the "dip" when I checked my wrist computer or gauges.
Any action cams are small so more affected by currents, bumps etc. than other video cameras. I can almost always tell when someone is using a mask mount. Often a pole mount also since the footage bounces more when the pole is fully extended.
How well does her regulator vent to the sides of her mask? That can also be annoying to see streams of bubbles in the footage every time. Remember that the GoPro's shoot 170o coverage standard with fixed focus and no zoom so they're very wide angle.
If she just wants to play around it will work - if she wants to produce something of more quality, buy her a tray or better mounting option. Z gear sells a BC mount that looks interesting also.
Obviously the 4Black shoots the highest resolution video. It may have more features than you want unless you want to shoot in 4K for UltraHD HDTV's now or in the future. Or want to shoot 1080P/120 resolution.
For me, the current sweet spot in the 4 series is the Silver as it comes with the monitor back so you can actually frame and compose what you're shooting. For most people 1080P/60 resolution is what they want - it's the native format of your current HDTV if you have one. And it has the same 12MP sensor as the Black for better quality still photos.
Otherwise you can drop down to the 3+Silver but you basically give up the monitor back and 2Megapixels in the sensor. Depending on what you plan to shoot, that may not be as big an issue.
The White has the old 5MP sensor so image quality - esp. for still shots - won't be as good. So does the new "Hero" entry level. The major difference between them seems to be the White has WiFi - and the entry level is not removable.
I like the Sony's for their footprint but they don't have any sort of monitoring option while shooting. The LCD on the side is only to display camera functions/settings. There's also the JVC Addixion(sp) which has the integrated side monitor - personally I'd find that weird to use.
