Well, I have a pretty strong reaction to this, and that is that this is a very, very bad idea and practice. The out-of-air diver (I assume she is OOA or why would she share air at every safety stop?) is learning really bad habits, which is to say she's learning to dive her tank dry and then always rely on someone else having air. This is blindingly obviously a recipe for disaster. One day, she will look up and her buddy will also be out of air, maybe because of fighting a current or a leak or what have you, and then what?
I say this as a self-confessed air hog photographer. The thought of planning to share gas just never occurred to me, nor should it have.
If she is not OOA, then I still think this is a bad practice if done more than once or twice, FOR practice. It sends the signal to everyone else in the group that there is a safety situation, which can then create its own issues.
Just a terrible practice IMHO.