How mobile is she? I'm assuming this would be the cheapest option but if she can do a little bit of walking and she's not carrying around heavy luggage, you can drop her off at Union Station to take the commuter rail in ($12.50 one way on a weekday, $10 on a weekend) and then at South Station she can take the Silver Line (+$2.40 for subway fare) straight to her terminal at Logan. I've done that method a few times in the past but I am in my 20's so it works ok for me as I can get around pretty easily and don't mind slightly longer travel time if it means I don't have to awkwardly interact with a van / car service driver haha.