Let's say I have a Pact of the Chain imp who is invisible and takes the Hide action to gain extra protection (harder to direct attack rolls to an unknown location). Then, he uses the Help action. Is his location still unknown since he has not left the heavy obscurement / invisibility that allows hiding? Or does Helping reveal the location? Or would it depend How they help (can it be done in a way so as to not reveal a location)?
It depends on what is the imp is doing (but the answer is probably "it breaks stealth")
Are they flying around a foe, trying to distract them in the middle of battle? They will no longer be hidden. They will still be invisible, but their location will be known (fluttering in the air next ot the foe).
Are they holding a lantern, directing light on the trap you are trying to disarm? They will no longer be hidden (everyone knows where they are by the light source they are carrying).
I can't think off the top of my head anything helpful they could do that wouldn't break their stealth, but there probably are examples.
As always, begin and end with the fiction (a line from Dungeon World). The player says what the imp is doing in the game world, and the GM rules from there.
Until you are discovered or you stop hiding … PHB p.177
That’s what ends hiding - being discovered or stopping; Helping is not mentioned. If you can Help without doing either of those things, you are still hidden.
Hard to see how you can find a way to Help without doing one of those things but all power to you if you can.