It seems to be that there is sage advice that one doesn't need to stay next to the target. I still feel like they need to stay there. It's good to remember that these tweets here and there aren't official.
Edit: There is official ruling to this later in the thread. It states that you don't have to be within 5ft when the attack happens.
"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." [PHB p. 192]
I find the "attacking a creature within 5 feet of you" being the key. If he attacks and the creature isn't 5 feet of helper the requirement is not met.
I understand that it can also be read so that you only need to be within 5 feet when you target the creature that you want to help against. To me it would need to be worded more clearly to say anything about targeting a creature within 5 feet and then explained that against the target that you selected the friendly creature gains the advantage and thus the requirement of 5 feet applies only to the targeting and doesn't have anything to do with the actual attack.
PS: Shows me how badly i remember rules. I remembered that help could only be used to gain advantage on melee attacks.