Borrowing from a known aquintace

[Gabriela Gomez]

I am planning on asking a person I know to borrow money for real estar investing. That person has 80k sitting at the bank and is planning on putting it into a secured annuity that will only generate her about 2%interest.  

With that said do we have to make it extremely legal or can it be just a promissory note that acknowledges that I will pay her back monthly at about 8-9% interest. I do not have the properties yet but I've got my eye on many that I can easily get cheaper if I pay cash. 

By the way it's not from a relative but from a person I know . 

Thank you in advance for your time.