My girlfriend is in the process of changing to a new supervisor and she had pass 1st year vivas with her previous supervisor.

With her change in supervision, her previous supervisor says she must not use any of the data generated in his lab in any shape or form for publication or communication purposes without obtaining his unequivocal and written consent. Is this a policy or is her supervisor referring to courtesy?

Can my girlfriend's previous supervisor deny her permission to use her results to write up in her thesis, if she had passed 1st year viva with the results, and what if some of her original data is being used by her previous supervisor in a journal manuscript that will be published?

I should imagine that this was intended as a reference to courtesy. Without any explicit disclaimer before the work was conducted, imposing that sort of condition after the event would be an interesting case to raise a grievance (or lawsuit) against.

Might we presume that, if your girlfriend is switching supervisors, there have been issues with the conduct / professionalism of the previous supervisor? In which case, it might explain why the message was worded / communicated as poorly as appears to be the case. Of course, I would hasten to add that many academics don't have the most developed interpersonal skills in the world, so it could just be ineptitude and inexperience on the previous supervisor's part...

Whilst technically the data belongs to the lab (she probably signed a intellectual property agreement), usually labs don't hold on to this when their students leave, because any publications should have the student and the supervisor as authors. This isn't a real policy though, and it's up to the supervisors what names are on the publications. Or up to your g/f if she decides to publish it regardless.

Also, unless she is changing her PhD topic, I don't see how she can get her PhD without the data, surely she won't be able to collect a extra year's worth of data without extending the duration of her PhD?

She should talk to the pastoral academics or the head of the graduate school for advice.