I wouldn't be embarrassed.....your friend should understand that cats have there own way of doing things! Maybe your cat sees something in this person that you as a human don't??
One of my cats never greets new comers. She is selective of who can catch a glimpise of her! Can't say I've ever had a cat that was into protecting the home front like yours did! What the heck chalk it up to strange yet ingtriging cats actions!

enjoy