It is a bit odd but I'm sure she had reason that was important to her. Not sure it would have been reasonable to ask her to move back and I certainly wouldn't have.
Empty seats in the ticketed cabin are pretty much fair game for anyone to move to and I'm pretty sure cabin crew wouldn't object if asked.
(Not relevant here but I think this doesn't apply on some planes when part loaded as there can be issues with aircraft balance if for instance all the pax move to the back.)