Is the pattern on your girlfriends horse only on one side as well? How strange that 2 horses, states apart but related by relationship of owners would get this. I hear the twilight Zone music right now. Ha ha
<-- This! Na na na na, Na na na na...
OP, that coincidence is alarming. I have never seen a horse develop an issue like this and I have no idea what could cause it so I'm no help there, but I hope you can figure out what's wrong and that nothing bad comes about from this unusual occurrence.