There is nothing wrong with it being called a 'theory' A theory is a set of statements or principles used to explain certain phenomena or facts. The emphasis is on facts, there. A theory is the term given to describe something that is generally accepted as true: ie, the theory of gravity, or relativity, or evolution (the last of which may or may not be accepted as true by a subset of individuals in the US but in other parts of the world is generally recognised as fact).
That said, I don't think that is what the person in question meant, from what you have told us here. It seems like you were saying she does not 'believe' in it.

No matter. She can't even use English appropriately, so who could expect her to understand DID.