It’s fine for premeds not to major in science as long as they complete the prereqs. Are you saying that the med school prereqs aren’t offered at her current school? Why would she need to take them elsewhere? Does the mention of taking them elsewhere mean that the aforementioned high GPA does not yet include the tough premed science classes?
School rank does not help with med school admissions. If she can maintain her high GPA where she is, including completing the prereqs, then she’s better off doing that (at least vis-a-vis med school admissions) than transferring and taking a hit to her GPA, no matter how “elite” the transfer school may be perceived to be.
I would say that if she wants to go all-in for pre-med, she should stay where she is. If there is a highly-regarded undergraduate major that she wants to switch into at the transfer school, that will launch her career without med school, that could be a legitimate reason to transfer. But I’m not reading that this is the case.
And yes, as others have said, if transferring causes her to take longer to graduate, because of credits not transferring or requirements not aligning, that could easily devour the cost savings.
I would lean toward “If it ain’t broke, don’t fix it,” unless the reasons to transfer are compelling.