Dillard is an English surname but, it could be (and evidently did since it is a friend"s name) made to become a first or middle name. It was not uncommon for people to use a family surname as a middle or first name by many people who descend from the peoples of the British Isles.
I knew a man once name Dillart, it could be the same name to, but he was German
and in German
d's are prounounced as t's. I think the deriviative of the "dill weed" a pickling spice could very well be where it comes from and likely it actually started out as some nickname for a specific person in orders for indentification amoung other people, and came to be a surname associated with the descendents of that family. Perhaps they pickled food for their livelyhood.