This kind of problem occurs because the strobe's lighting is too different from the ambient lighting -- the subject looks she was is a different location with different lighting than the rest of the scene. In particular, the subject looks like she should be in shadow if she were sitting on that wall but she is not.
Generally, a strobe in these kinds of outdoor situations is meant to:
1) replicate the ambient lighting where is unavailable due to shadows from out-of-frame objects.
2) soften but not eliminate harsh shadows created by a strong-direct ambient source like the sun.
In this case it looks like:
1) The strobe may be too bright and overpowering the ambient or removing too much shadow
2) The strobe may have the wrong angle - creating different shadows on the subject than are not in the rest of the scene
3) The strobe may have a different color (in this case, the sunset's reddish light is different from the strobe's blueish light)
4)The strobe's light may be too narrowly focused -- the strobe doesn't seem to be lighting enough of the background/foreground to help integrate the lit subject into the scene.