You can't spoil a baby by breastfeeding them, especially by letting them nurse to sleep. 

The Breastmilk Queen - Amy McGlade says, 

"Mama never feel guilty for breastfeeding your baby for comfort or as they drift off to sleep. This is normal, healthy, and developmentally appropriate. Most babies nurse to sleep and wake 1-3 times during the night for the first year or so. Some babies don’t do this, but they are the exception, not the rule. Many children, if given the choice, prefer to nurse to sleep through the second year and beyond. I’ve never seen a convincing reason why mothers shouldn’t use this wonderful tool that we’ve been given."

Nobody should be able to interfere with a breastfed child's need to be with their mother at night!!