Hmmm.....Well, if he's still that tiny and he's not eating enough on his own, then she's doing the right thing. Is there any way to change your vacation?
Since he's not being shipped, and you'll be right there with him in the car, I think that with the proper instruction on his feeding he'd be absolutely fine. I would just get the feeding info from her and keep the plans the same if it was me. As long as you are aware of the signs of hypoglycemia and how to prevent and treat it, then she should trust that you will care for him properly.
Talk to her about it some more and let her know that you are knowledgeable about these things and she may change her mind
Good luck and please keep us posted.