I wouldn't blame your friend. If he's in love with this girl he should respect her wishes. I strongly feel that dating is a path towards marriage and as you get closer to marriage your potential spouse should be your best friend. As FlDebra said "someone to share everything with". In this world you can only be fully transparent for very few people. If you love your friend, the nicest thing you can do for him is leave them alone. In time the girl will see that you aren't a threat and possibly you will be able to double date. You cannot hope to be as close to a guy in a relationship as on that is single.
Though it is a silly thing and I don't necessarily agree with it:
The Ladder Theory can explain what and how guys think. (generally speaking, guys like yorkies do not always follow the standard and there are many many breeds out there)