I know with GCSE and A level Art they have a certain number of hours in the classroom that they can spend on their coursework. They can prepare for it in advance, at home, and they can still work on research etc at home whilst the hours are in progress, but the work cannot be taken home and worked on there, for obvious reasons. It sounds like BTEC is similar.
The thing is, though, that it is the teacher's responsibility to make sure nobody takes work home during that time. It's like an exam room - you make sure nobody brings in or takes out anything.
I can't see this affecting subsequent GCSEs. It would be different if, say, she'd taken notes in to a History exam; the teachers might suspect she'd done it for other exams but hadn't got caught.
This seems like a misunderstanding and the teacher hasn't policed it well.
I'm surprised the school hasn't contacted you. As someone's said, the LAST thing they want is someone to fail; despite some people's opinions, the teachers do want the students to pass, if only for the sake of their own reputation. Even if someone's been a PITA throughout the last five years, the teacher still wants them to pass.