when the door closes make sure you hear the audible double beep beep with the amber lights flashing then this means it recignizes the key fob for auto lock and when you walk away it then should beepn again with the amber lights.. i do this with mine and this way i know its always going to lock.
I'd second Detailguy's advice. It might also be worth checking whether the auto lock function is selected as part of your wife's driver profile. If you are each using your own key, then it could be that she hasn't got it included as part of her setup.
Also has your wife demonstrated the no lock scenario to you? It would be worth observing her exiting the vehicle as I find that sometimes a rapid exit means the auto locking process doesn't initiate.