Sounds like 'Mommy Dearest' just wanted someone else to blame.
It's not your fault her precious Snookums doesn't know how to properly care for her own horse, and as her parent SHE'S the one responsible for her, not you.
At 13 y/o, that kid isn't a baby anymore. If she didn't call her mother and said she did, why should you have to toddle around behind her?
Next time, I would say you should get the kids cell phones and dial their parents numbers for them, since they clearly don't have the responsibility to do so. Nor to care for their horses.