@The Pentacle Queen,
I agree that's tough.
I don't know, if it were him asking this, I'd say "she's giving you every indication that she likes you a lot as a friend and respects you as a business partner but is not sexually/romantically interested in you." Then I'd tell him to get over it and look for someone else to focus on.
But, he's not here.
I generally am for openness but from what you say I don't think that'd be the best thing here. You both have plausible deniability right now.
I think you're doing the right thing by regularly mentioning your current bf (or whateverheis), although the whateverheisness might be part of the problem? Your friend might not think that you're really involved in any meaningful way.
I think a certain amount of management is called for -- avoid late night bar situations, the two of you alone, that sort of thing.
If it was any other sort of relationship I'd probably advise just putting all of your cards on the table. But I understand that you really don't want this to go south, and "you're in love with me but I'm not attracted to you," in pretty much any overt permutation, will most likely take things in a southerly direction.