Being the queen of stupid questions, can I just ask..... Why on earth did your Mum's friend by this mare in the first place??? From what you has told us so far, she can't handle the horse, isn't horsey and therefore doesn't know what the horse should be fed, when it is sick and when a vet should be called! What does she plan to do with this poor horse.... leave it to rot in the paddock?
It's sad that you have been put in this situation and you are doing all that you possibly can do. You've gone well above and beyond your responsibilities here. Well done to you, and a big bah humbug
to the horse's owner!!!