iphone5 using cellular data watching netflix when i'm connected to wifi

Hello, my mother in law has an iphone5 using iOS 8.4.1 and she recently connected to her neighbor's wifi so she can watch netflix (as well as web browsing, email, etc). She was eating up data fast so she got permission to use upstair neighbor's wifi. However, even after successfully connecting to wifi she continues to eat up data watching netflix. I have tried the following things:

I power cycled her iphone. Reset her data usage statistics back to zero. While connected to wifi watched a 5 minute youtube video. Checked data usage and it read zero. I turned off wifi and watched another youtube video and it showed data usage. Turned wifi back on then "assumed" everything was ok.

The next day I checked her phone on the AT&T account and it shows its still using data. I called her up and worked with her and I had her delete the wifi connection and re-add it. She checked her data usage and it read 336MB. She then watched a netflix show and checked the iphone data usage in the settings and it then read 600-ish MB. I verified with her that the hotspot was turned off as well.

I understand I can ask her to turn Cellular Data off in the settings menu whenever she wants to watch netflix so it will use wifi, but that's just a band-aid fix rather than a real solution. I understand past issues people have noted with iphone going into a sleep state and wifi turning off which can use cellular data, but this isn't that scenario. She's actively using her phone watching a show and it's using data instead of wifi.

Is this a similar problem others have seen? Is there a solution to this or is it just a matter of either rebooting iphone or delete/re-add wifi connection being only "fixes"?

Cheers!