@Johnnsmith,
You're thinking I think if you don't say something she will continue and get engaged etc and you'll lose her.
When someone is in a "committed" relationship, it has to run it's course. Be it, that they get married or break up. Taken means taken but I am one to believe that you should be true to yourself and others.
I would simply state to her, "You're a great friend, there are a lot of things I like about you, if you didn't have a boyfriend who knows"....
Leave it at that, as you've basically not committed yourself to stating anything major and on the same accord you've given her food for thought, she can take that paragraph two ways and the ball will be in her court.