Instead of fill flash just overexpose a bit... especially with her skin and make sure the ground underneath her or just in front of her is reflecting light onto her. (like a reflector or use an actual reflector). Unless using a softbox a fill flash looks like garbage IMHO.
-Some poses are absolutely not flattering and you can only figure it out by doing it... In one photo she's squatting to go to the bathroom it appears
-In one a single leg is behind her where she looks like an amputee.
-In the picture reposted above her arm is being flattened by something making it appear larger than it really is. Try to minimize this. Girls like looking thin.
-Many of the pictures appear to be taken from a low angle... get to eye level or even above eye level looking down to make her appear a little thinner.
-I don't see any close ups of her face? What does she really look like?
-Many shots appear soft or downright out of focus, make sure that shutter speed is at least double the focal length, lock that focus and hold still. With that 50 all you need to do is sway slightly and your focus is gone. You might consider AI Focus/AI Servo.