score:2
Not having the passport (and the number) anymore should be a perfectly fine reason to leave it blank.
The assumption is that passports usually are valid for ten years, so you would still have a valid passport - but if not, fine.
Upvote:1
If she has applied for an e-visa, or bought tickets on certain airlines she may find the full passport number in her e-mail cache, especially if she uses gmail and never deletes. Some foreign financial transactions might also require that info.
Sometimes filled out .pdf forms for visas etc. are also present on one's computer (I save mine in a dedicated folder).
For example, I found my ca. 2014 passport number in my email on reservations from Emirates, Hainan airlines and on a couple e-visas.