Well, think of it this way. He's never had a girlfriend, let alone a real one, and he finally has one. And as you said, she's really nice. So, as they always say, put yourself in his shoes. Ask yourself who you'd rather spend more time with; your girlfriend or your friend. I don't know if you have had a girlfriend before or not, but in the case that you fall in love with you she becomes a part of you. That is a much tighter bond than you would have with your best friend.
Basically, you end up with two best friends. One is a part of you, and the other is simply your friend. If that makes any sense. While you may feel that it infinges on your friendship, you also have to consider what you would do in the same situation. In that aspect, I don't think that girlfriends/boyfriends ruin other friendships (but they can in other ways, but that is a different topic for a different time).