The Effexor won't help. It can take weeks to completely build up in your system. If I were you, I would call her Dr and tell them what's going on. If she's been taking Xanax for awhile and on an everyday basis she can go through withdrawal.
Another thing is that xanax is for anxiety while Effexor is for depression.
Effexor works for anxiety too. I was on it myself for several years. But, it can take weeks for it to start working.