It's kind of weird for Lily to totally forget the other man when she was just before observing both of them together and realising they meet up regularly.
@tigerstar186 The fairies' magic is weak, so perhaps the man's mental state needed to be vulnerable for the weak sleep spell to work. They tied him up in case he wakes up too soon. Normally people don't just fall asleep randomly in the middle of an operation, so if someone had found him sleeping in a room he was supposed to be checking for servants, foul play would be suspected anyway. It would be a different thing if they were merely on guard duty, like told to watch a gate for two hours in the middle of the night. Then it wouldn't be unheard of to fall asleep.