I'm going to assume Nandor wished for Guillermo to want to be human again. He used the little lamp, not the big one they tried to dupe us on this ep.
I simply don't buy that Guillermo would have chosen human over vampire without a shred of consideration.
If Nandor knew that Guillermo would choose to be a vampire, he wouldn't have set up the elaborate ceremony that included Derek. He did it because it takes all blame off of himself and gave him an out on having to kill Guillermo without looking like Guillermo won. And, you know, it also reopens the opportunity for Nandor to turn Guillermo himself and them to live together forever.
And if Nandor would have outright forced him out of it, Nandor knows Guillermo - and potentially Nadja and Laz - would never look at him the same.