I was taught that the player over whose head the ball is going should make an immediate decision on whether they are or are not going to play that lob. If so, they say "Mine." If not, they say "Switch" or some such.
Since none of us are perfect (and because we often play with others who don't do things this way), the deep person can make the call and yell "Switch."
I doubt that anyone taught your friend that when a lob goes over her head she should do nothing and stand there in her partner's way.
I partner with someone who does this, however. Her problem is her overall court awareness. So she won't recognize that if the ball is directly behind her, that means her partner is directly behind her. If her partner tells her to switch, she won't do it unless the command is given quickly and will claim that there was no time to switch.