How Do I Partner with the Seller on a Fix n Flip?

[Todd Rizzardi]

Hi. A friend of mine has a residential SFR with an ARV of ~$400k in Southern Cal here where we live, and is asking me for help to fix n flip so she can cash out of the property. 

She owes $257K. I have the cash for the rehab which I estimated about $30k. 

Let's say for round numbers the potential profit is $100k. First I will take my $30k back so then there is $70k left over. I will factor in realtor commission etc afterward.

First Question: What is a fair way to split the profits with her? Considering I am putting up all the money for the repairs and managing the rehab. 

Second Question: How do I write up a contract for something like this since I am not buying the house? How do I secure my position? Does anyone have such a contract I may use as an example to structure this type of deal? 

Any other suggestions are most welcome. Thank you kindly for your support.