The complaint is at odds with my answer.
Your GF has a round face, the common technique would be to slim the face by throwing more of it into shadow, google short lighting. Bt you really need to find something about face shapes and lighting, it's a fascinating subject and also includes posing angles and camera angles to make the most of it.
Heres a
quick find.
The large flat soft lighting that's become 'normal' doesn't really flatter anyone, and when I was learning photography was only really recommended for beautiful young women (read slim and flawless complexion). Like white backgrounds, it's become the norm and photography generally is poorer for it.
Oddly all photographers actually know better, if I asked you to post your 5 favourite portraits, none of them would have flat light or a white background. But ask 10 relativeely new photographers to produce a 'studio portrait' and 8 of them will go for a white background and fairly flat lighting.
I'll stop now I'm starting to sound like Garry.
As expected, I agree 100%.
Let's go back a bit...
I would be very interested to hear how others may light the image to be a little more flattering?
More flattering in what way? What are you trying to achieve here? What's the purpose of the photo anyway?
If you want it to flatter her in the sense that you want to show her as a nice, kind person who is friendly and sympathetic, then you've done a very good job.
If you want to flatter her looks though, there's room for improvement. As Phil says, go for short lighting if you want to really slim her face, there's nothing quite like it if that's the effect you want.
Photography is a 2-dimensional medium that depicts 3-dimensional subjects. That's why people tend to look much rounder in photos than they do in real life. If you want her to look as slim in the photo as she does in real life, you need to light some parts of her more than others - so don't light the sides of her face, as you have done here. Short lighting involves having the face at an angle and only lighting the part of the face that is least visible to the camera (the opposite of this is called broad lighting) but another way of doing it is to have the light high and directly in front of where her face is pointing, this will draw attention to her cheekbones and leave the sides of her face unlit, or lit less brightly, which will slim the face a lot. It will also place the catchlight in the eyes higher, which will make the eyes look bigger.
If you want her to look sexy, don't get her to smile. You will NEVER see a shot of a sexy woman smiling, partly because the smile creates the creases and shadows that she's complaining about, and partly because when people smile they look nice - people want to talk to nice people but they don't want to take them to bed
If she can do it whilst looking natural (and a lot of people can't) get her to have her lips slightly apart, but not smiling. This makes the lips look bigger, with obvious sexual connotations, it creates that essential little shadow under the bottom lip too.
I wouldn't use a fill light for this type of lighting, I would put your speedlight on her hair instead.
And I wouldn't use your reflector either. I would get a sheet of A4 white paper, fold it in half (A5) and get her to hold it just out of shot, so that it directs a little light under her chin only.
Try it an see. There will be shadows and you may find your clothes in a bin bag outside the front door, but I think that you'll like it.
This is a fairly extreme example of what I mean. You'll either like it or hate it
Whether you like it or not, it illustrates what I mean.
BTW, not my photo. Michael Sewell did the clicky bit, I was just there as general factotum and labourer, and I like to make 'helpful' suggestions on the lighting

This shot was taken for a particular purpose and fits the brief well. It isn't your average family portrait, but the principles apply