To me it's very hard to put a dollar amount or percentage on a true friendship.

Try and establish a friendship with this ¨most trustworthy seeming¨ investor and maybe help them out with a flip so you can learn the business and hit the ground running when you turn 18. 

Perhaps avail yourself as a free intern for a period of time to help the person out while all you get is the information from doing it (you can also make good friendships that way). 

Really, you have to put the friendship first.

@Bill Piker, it is a business not a friendship.  

I hope that this proves to be a long and fruitful friendship/partnership for you two guys and everyone who worked with you on this deal. 

I wanted to go for it, but worry about loosing friendship/money/relationship/trust if something goes south.I understand we need LLC or some entity as well as partnership agreement is good start.

I have made great partnerships and friendships here.

I joined bigger pockets to network, learn, create friendships and form win/win relationships.If you live in my area, let's meet up for coffee sometime!

Not to mention the friendships and business relationships i formed thanks to BP.