It sounds like the conference reimbursement would be allowable to the sponsor, and allocable in that attending the conference was beneficial to the project. So the question is, is this reasonable.
I would say it would be reasonable if she was working remotely for majority of the project, or working remotely in Virginia because there is was a benefit to the project, etc.
I would say it would be unreasonable if she was in Virginia because of a choice of her own.
You probably want to get additional justification from the PI and grad why they feel this is reasonable, to help make your determination.
If you're leaning towards unreasonable tell the PI that you need sponsor approval before you can approve the expense. If the sponsor approves then I would approve.