"I forgot who she wants to talk to."
J'ai oublié ____ elle veut parler.
What should go into the blank? Ce qui, ce que, qui, and que are out, because parler requires the preposition à in this context. Ce à quoi doesn't work either because we have a person here.
The sensible choice seems to be ce à qui, but is this actually used? Otherwise, can we use à qui?