My internet has gone down on random occasions, and I use my cell phones hotspot feature. I can not get a screen cap at the moment but using it for two days straight it used way less than a gig. I have the 10 gb 'unlimited' internet plan though that just means 10 gb and after that you get super slow speeds. I was honestly surprised how little it used, and was also very pleasantly surprised because if I were ever away from home I could use it for me and my partner as temporary internet if there was a long term issue. Now netflix or youtube, aye that would eat the data super fast.
Though no offense to the op or their partner, but having situations similar occur in my past... It just sounds like an excuse to start a fight. I could be entirely off base with this assumption, but I truly hope this is not the case and they could see reason. There are numerous ways to track data usage over the network to know what device is using how much bandwidth.
You can check it within your router if you know how to access it, unless having the standard modem/router combo net providers sometimes rent out. There are a few articles that could help for how to show them how much it is using. It is tough to assist completely without knowing if the other party will understand or listen if given evidence in the first place.
Heck it could even be someone sneaking into your wifi or a phone going rogue. When I was on wifi my all in one Lenovo decided it would download a ton of windows updates in the back round I had to go into my router and hunt down which mac address was causing the massive data usage all of the sudden. Also believe it or not a downstairs neighbor at some point had their huge microwave placed directly below where I placed my router and it would cut out every time they would microwave anything literally getting kicked from final fantasy mid dungeon.
I wish you both well, and hope you find what is zapping your net.