What kind of hay is it and I would feed the mare some grain along with supplements. She is going to need them now and when she does foal.
I usually don't sweat about a mare foaling by herself but in your case with this mares condition I would really try and be there in case of any emergency.
Just because the sire had good bloodlines and is sweet natured does not mean the does not carry something like HERDA, HYPP or lethal white.
If he is only a carrier and shows no signs he might have passed this on to his daugther. Toghether the two have a greater chance of passing along something than two non related individuals. Good Luck Shalom