I've signed NDAs as part of my employment contracts - as others have said, I get a job out of it.
However, when there have been specific projects where they've tried to impose extra NDAs, I've refused and said my employment contract (and my duties to the court as a solicitor) are sufficient.
If I knew a famous person, I would be rather flummoxed by the "requirement" to sign an NDA. About what? They can't impose a duty of confidentiality retrospectively, because it's unlikely that previous information disclosed/that the OP knew would be confidential anyway (eg family stuff, things that happened at school, an affair she might have had), so all they'd be doing is saying "I won't tell you about x project I am working on unless you sign it". At that point you'd have to decide if you want to know about x project.
When you sign a confidentiality agreement, the information protected has to have the "necessary quality of confidence". I can't see that that is effectively gossip has that quality.
I would not sign it unless I was going to be asked to be involved in some sort of project, and then I'd expect it to come from the media/production/publishing company etc, not her. And of course, as others have said, you have to get something in return eg money. Although there is an argument that being involved in a project could be "payment" enough.