There are several issues here: 1) Having a relationship with a subordinate is not normal today. Planning to marry a subordinate that one has been dating for on

There are several issues here: 1) Having a relationship with a subordinate is not normal today. Planning to marry a subordinate that one has been dating for only a few weeks isn't normal at all. Your co-founder's behavior with a new employee opens your company up to harassment lawsuits by his fiance if the relationship fails, and by other employees whatever happens to that relationship. 2) Being a co-founder wouldn't give her the right to make decisions for the company. It's a label, not a meaningful legal title. 3) She is not a co-founder and should not be made one if the business already exists as a legal entity. You can consider bringing her on a a high-level employee/executive, since it sounds like she has already done executive-level tasks like bringing in partnerships, but that is something you need to discuss with a lawyer. 3) Your co-founder and fiance already making decisions without you is a huge red flag, and it is likely that after they marry they will formally push you out of the company. She is not a co-founder, or executive, or even a long-term employee. She's literally someone he met a few weeks ago. She shouldn't be involved in decision-making at all.