Well Year One is a good company, but they do not stock many parts. Many of the parts you order from them will take a little time because they first have to order them. I guess you could label them as a parts reseller. Their parts are usually of real good quality because they order from some of the better companies.
I do not know why the mustang parts are taking so long, but the Ford parts are the newest addition to their lineup, so it may take a while to get them.
Your friend should be able to get a refund since it is Year One's fault for misleading him on the delivery date.