The way that I understand it. There is something about a tendon and muscle that spasms over a long period of time and shortens, thus changing the angle of the coffin bone (or something, it's late and my brain is only half awake). Here
is the website that I found and one of the illustrations looks very much like your mare.
I bet that she does have a moderate club, possibly from an injury suffered on the track. Has she had this since you got her or has it developed during your ownership? Of course, without actually seeing the mare and her feet, this is just a guesstimation but....It looks to me like the "dip" is where the angle of her hoof growth changed. That would be a decent indication of how long ago the issue started. Since the hoof above the dip appears to be growing straight, even if it is different than the hoof below. The dip does not appear to effect the hoof above. So, since it appears that it is a fairly recent development, I am willing to bet that she will begin to grow higher heels as the growth progresses and the hoof wall evens out into it's new angle.
But, keep in mind this is just my uneducated opinion based on one photo
. You might have your friend get your vet and farrier together and see what they can come up with.