As written, can a flying familiar fly to within 5 ft of an enemy, use the Help action to give advantage to an ally's next attack vs that enemy, then immediately fly away out of reach using the Flyby ability to avoid any possible attack?
You can break up your movement on your turn, using some of your speed before and after your action. For example, if you have a speed of 30 feet, you can move 10 feet, take your action, and then move 20 feet.
You can lend your aid to another creature in the completion of a task. When you take the Help Action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn.
Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective. if your ally attacks the target before your next turn, the first attack roll is made with advantage.
Flyby. The owl doesn't provoke opportunity attacks when it flies out of an enemy's reach.
As you can break up movement around an action, you can move both before and after taking the help action (up to the limit of your movement speed). Movement, the help action, and the flyby ability don't enforce any restrictions between one another, so the familiar would be able to take a portion of its movement, use the help action, and use the rest of its movement, benefiting from the flyby ability in both portions of its movement.