Dictionary definition of owner says that ‘a person who owns something is known as owner’ and dictionary definition of co-founder says that ‘ a co-founder is a joint founder of something’.
Going by both the definitions, I think that a co-founder could also be owner if he has fully invested capital in the business that he is a co-founder of.
But if he/she was recruited at the position of co-founder than he/she cannot be deemed as an owner.