I'm purchasing a duplex on a 30 year traditional mortgage.

I have been pre-approved through the traditional means (20-25% down) but I have been recently offered, by someone I trust, to borrow money from them using money they will pull out of a HELOC on their primary residence.

You'll still be ahead of the game by generating income from it.  

On the other hand, if you are accredited, then there are many other choices with much more experience, much more skin in the game and/or much lower fees.

So trust me, its just the name of the game.

Also, ensure that the flipper will still have skin in the game.

Yes but there is no easy road to success.This game is a marathon and if you remember that you won't get discouraged with the ups and downs,,steady as she goes and you will get to your goal.

It is not a home run deal, but it did get us in the game.

One company owns everything and does nothing (this is your SLLC a/k/a "asset holding company") and a completely separate company handles all of your operations (this is a traditional LLC a/k/a "operating company") For the operating company which serves as your face to the world and through which you do all your business, you establish a Traditional LLC to carry out the operations of your investments.

Also, depending on your business model, and how you’re playing the long and short game, you can be drastically affected by the drop in the market.