In hindsight, I should have just fired the designer. But I was trying to be a good partner and go to him with my issues first. He agreed and said he would take care of it.
I think I've downplayed how much power he's given this person. Not just design of marketing stuff, but actual product UI design. Plus a lot marketing/budgeting decisions, etc. Basically this person has become or directed the entire face of our company. For whatever reason, despite no sales growth, my partner hangs on and defends this person's every decision.
If it matters, I'll add that this person is a woman. My partner has assured me that they are not in a romantic relationship and have simply become extremely close friends. I believe him–but many people on the outside get the impression they are romantically involved.