I've only ever used 3 season sleeping bags in bothies that go down to -3°c and I've been fine. Much depends on the weather at the time. It's key to eat plenty to keep you warm and take clothing you can sleep in if it does get a bit chilly.
You'll want to ration your fuel for evening use only because between you both I doubt you'll be able to carry enough coal/logs in to burn all day long.
Bring candles with you too and a wee Bluetooth speaker and she'll hopefully love the experience.
NB. I wouldn't go to the expense of buying expensive insulated mats for bothies. Bit OTT.