In a case like this a long-time interpreter told me that she usually approaches the deaf person and instead of a) jumping in to interpret or b) asking if the deaf person needs an interpreter, asks the deaf person whether the hearing person needs help.
The only time I can think of that I would jump in to interpret without asking is in some sort of emergency. Generally my priority is not to behave in a patriarchal manner but if there were a life at stake I'd probably let that go.
