Good on realizing you’re too heavy. If I were you I’d just ease into IF if you really want to succeed. No reason in fasting for 24 hours in the beginning. That’s also not how you want to sustain your relationship with your girlfriend I suppose.
Why don’t you start 16:8 for a month or two. Having a window from 12:00:20:00 you can have a morning tea or black coffee and also share lunch and diner with your GF. By doing this you won’t snack after diner and will lose weight easily. After a month or 2 I can assure you’ll be sub 100 kg’s. By then you have a way better understanding of how IF works and how your body responds to it. Perhaps then you can be more aggressive by doing 18:6 , 20:4 or OMAD.
Doing this you can ease into IF and also let your GF ease into it.
Good luck
Google TDEE calculator. With a few inputs from you (current weight, height, age and activity level) that will give you your maintenance calories and should also give you other numbers for when you want to lose weight and when you want to gain or bulk up. Or you can just subtract 200 calories or so from the maintenance number. Good luck!
While I don’t know you or your situation, your girlfriend has a point in saying it is too extreme. I would really, really suggest consulting a doctor about this beforehand. In my view, this is walking the line a bit between fasting and an eating disorder.