Its basically something to do with the mare's blood type. Her colostrum can cause the destruction of red blood cells and kill the foal. They say not to let the foal nurse for 48 hours. You would need a milk replacement (like powdered foal milk) and an antibiotic for the foal as well a s a muzzle to prevent nursing and to milk the mare and discard the colostrum. Im attaching links that talks about it.
If you do breed this mare Don't let the foal nurse. You could degrees the chances of an issue with it if you blood type the mare and stud. If they are a match it may not cause NI? IDK I would consult a vet if you did decide to breed her. I would avaoid it if I could though. http://extension.vetmed.ufl.edu/file...-the-Horse.pdf Foal Neonatal Isoerythrolysis | iGrow | SDSU Extension