If she is done with her requirements, go ahead and use the potion since it's allowed.
You're using friendly interaction to get the move in option right? You have to be using only friendly interactions, no romance options to get the "Ask to Move In" It usually comes when I'm around the "thinks So&So is sociable". Get some of the other Asks out of the way like, your day, your alma mater, your job if you haven't already. Just remember to stay in the friendly interactions.