Charles Helliwell's Answer:
Honesty is the best policy all round, in this instance. You'll have to come clean with your colleague and tell her where you stand. However, there's an interesting angle to your story, because you are both friends and work well with each other. So, why don't you both see whether or not you can exercise the judgement of Solomon on this and share the responsibilities of the creative role?
You have shown, for example, that your skills lie in the area of organisation and planning. Is your colleague perhaps more of a creative influence on the partnership? Many a successful advertising and marketing agency has been built around the partnership of two individuals whose responsibilities are shared and whose skills are complimentary.
Perhaps one of the ways that you can demonstrate this partnership is in the allocation of the offices. Your colleague wants to have one of the offices with a window seat, which is being earmarked for managers in the business.
So why don't you compromise with her that not only will you share responsibilities, but she can also have the office with a window seat? In exchange, you will be given the title and responsibility of manager of the group.
I'd say that was a pretty fair bargain, in that you both get much of what you're looking for, whilst continuing to work together in a collaborative fashion and remaining as friends.
But beware in the longer term, because these sorts of compromises are only a short-term fix. Over time, your managerial responsibilities will eventually lead to a different working and personal relationship with your friend and colleague. That's the nature of business, I'm afraid. The judgement of Solomon eventually leads to one party coming out ahead of the other.