its not the best angle of photography!!
I would suggest looking at your friend's horse in its entirety first and ask - is it a relatively compact horse?
Then for your friend to ask of herself : does the stifle turn out a compensatory amount to the hind hooves?
If the answer is yes to both questions, I'd suggest (without seeing the horse in the flesh) your friend's horse is fine!
Your friend's horse's cannons appear to be running relatively parrallel in this photo. Could your friend or yourself after seeing the horse in the flesh, given this particular angle of photography and stance of the individual - would you say it's cannons are running parrallel? ie the space between the top of the cannon equals the space at the bottom of the cannon.Individuals with parrallel cannons can not be cow-hocked.
Cow-hocks is such a mis-used terminology in equines! (no offence, Liz - but I'd like clarification on the points I've highlighted before making such a decision and stating cow-hocks)
Also - given the angle of the photo, would you, C-Ma, suggest this horse's cannons are running straight down from the hindquarter (from point of buttock) ie don't 'poke out' (camped out) or deviate under the hindquarter (sickle hocked).
Where as a horse with a longer type body might not need the same amount of angulation of the hindquarter for tracking through!!
If your friend's horse is indeed compact with no turn of stifle, with "straight" hindlegs ie hind hooves facing straight, it is more than likely that its ability to track through would be lacking or non existent as the stifle would be hitting its abdomin and causing repitious aggravation/injury.
To have a turn out of stifle on a compact individual without compensatory turn in of hock, turn out of hind hoof would also be a concern for the individual, it wouldn't be encouraged to work to its best potential with its hindquarter due to being uncomfortable!
So, mho, it really is best to appraise and display the entire individual (from all sides, as square as possible) before focussing on an attribute!
And if you wish to appraise an attribute - start from the "start" or top in this case - the pelvis and work your way down
Usually an attribute(s) will show itself as a compensation! Understanding these compensations is the art of judging and understanding what an individual would be capable of.
Horses for Courses - Nature has made them this way, best we can do is learn to understand Nature.
No horse/equine is mechanically perfect
although we aspire them to be!