@bookworm: I agree with you.
In my opinion, this is a very bad start. Text the roommate & politely state that you need to be informed ahead of time about any proposed joint purchases. This seems as if the older roommate is experienced in taking advantage of another. Regardless of whether or not that is an accurate impression, it seems that she is inconsiderate & setting your daughter up to be the bad person if she stands up for herself and objects.