Leon can't leave Ashley when he transitions into a new area (signified by the blurring effect); however, he can leave Ashley safe in the corner of a dangerous area then call her over when its safe. You can instantly tell if Leon is about to leave an area if the OPEN or context command is printed in green. If the command is printed in white, Leon will simply open the door to another section of the area. Use this to plan out where Leon can leave Ashley to brave the dangers ahead.