I would say something...especially now that it's been a few days.
I would mention something like, "I happened to run across an article in a health magazine the other day and it stated how important it was for baby's to be held while being bottle fed, vs. being propped up, and it's given me a different perspective. I know you're experienced with feeding baby's, but after reading that article, I'd really prefer you to hold my baby while feeding him. I'm assuming you don't always feed the baby by proping him up right?..I just want you to know that that would make me uncomfortable and I just want to verify." She'll most likely agree with you and comply.
There really are health concerns to not holding the baby. For one, there is a choking hazzard because baby bottles placed in the mouth can fall down too deep, while emiting liquid and if the baby is too young, it could drown for lack of muscles to remove the bottle. There is also the air in the bottles, of which don't get expelled properly so that the baby gets more gassy and spits up more of the liquid, again, causing a choking hazzard if the bottle is in the mouth not being monitored by a person holding the infant. There is also a lack of bonding in which an infant needs no matter WHO is doing the feeding.
http://www.nncc.org/Nutrition/guide.bottlefed.html
http://www.babycenter.com/408_when-can-my-baby-hold-his-o...
Hope this helps.
there are probably better resources out there but these are 2 that I could find very quickly.