In battle, the Help action has a normal reach of 5ft. A reach weapon has a reach of 10ft. Logic says it would be possible to distract an ally's opponent by poking at him with a reach weapon, but RAW do not have a clause about that. So, is such an extended Help possible?
Assuming you are talking about the secondary aspect of help in which (PHB 192)
...you can aid a friendly creature in attacking a creature within 5' of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective.
The specific language here does require you to be within 5' and discussion of Reach or weapon isn't part of the calculations for Help. In fact, the examples given have very little to do with weapons themselves and you can still Help without holding any weapon at all.
But what if?
The language in the second sentence quoted above seems to leave aside the possibility of what you are describing. It isn't RAW, but it is reasonable to discuss it with your DM. I don't think there is a danger to this, but extending the 'range' of help may be a slippery slope - but the danger is small. The help action takes up an entire action and only assists on the next attack. If your other PC has multiple attacks, they only get advantage on the first. It's a large cost of Action Economy for less payback if you have multiple attacks.
There is another option that does something similar that I'd like to offer as well:
What you are describing here is more like the Flanking rules described in the DMG (page 251). If this is something that interests you, discuss utilizing it with your DM - however do note that a lot of abilities/actions in 5e require advantage to initiate and the easier you create advantage, the more common those things will be.