^^^ I agree with the fainting timing bit. That was a bit contrived - no argument there at all. I was quite disappointed at how that all played out.
The memory loss is a different animal entirely, I think. Renard is a prince, which seems to imply that he has special abilities that stem from his "royal blood". The use of spells and potions are pretty commonplace in this show now. It's not as much of a stretch to think that, if Nick is Juliette's "true love" and she is kissed by another with Renard's abilities, coupled with the poison that she got from the cat, it could follow that she would be forced to forget that true love through the confluence of precisely-timed events. This may have been the plan all along to put Nick off his game and leave him confused and vulnerable. Now that Hank is involved, however (something nobody knows at present but Nick and the Coyotl family), he can cover Nick's back more effectively and have a better idea of the weird stuff to expect.