It does look a little sore , but don't beat yourself up as these things happen occasionally. She has quite a couperose complexion from what I can see , so you are dealing with a typically sensitive client anyway. (like me for example!)
I do my own brow wax and on the odd occasion It has happened to me when I have been
Due a period and then menopausal - can't win
Been in the sun or used a sunbed....years ago, when I was stupid.
Used a facial scrub say 24 hours before treatment.
Used an anti ageing moisturiser that contains some mild vitamin based skin peel.
However on clients since it is so much worse than making a graze than on youself, some of the above factors also apply.
Medication can also affect the skin, making it more delicate.
In addition, doing it all in one strip , not applying talc or some therapists use oil ( I don't ) can also lead to this.
Some of my clients have the hard wax brow treatment and that seems to be better than strip wax, because it shrink wraps the hair without affecting the skin beneath.
Meanwhile ,I would tell her to apply sudocrem, just dab it on to a clean brow area and it should settle down in a day or two. keep in touch with her because you want to troubleshoot and sort the next appointment so that you work with her on how you do the next brow session.
You may just pluck for the next time, see how things go.