My friend is currently living with a woman and her two children. He bought and had placed on her property a mobile home with the intent of living there alone. While he was at work, she along with her children moved into the home. He has been cohabiting with her and supporting them her for a year now and has never had any intent to marry her. His fear is that she will file a common law suit against him. How does he ensure his intentions not to be married are documented.

First, any question on law needs to include a general local as laws vary.

I believe only one or two states still recognize common law marriage. If your friend wants to continue to co-habitate with this woman, then his best bet would be to get a Family Law attorney to draw up an agreement like a pre-nup that would protect his property.

Yes where are you at ? Very few places in the US accept it, so it makes a difference, after that each location will have some specific requirements.

If he is so concerned and does not want to be married, why does he merely not move out

From my understanding, common law only applies if BOTH parties agree to the "world" that they are husband and wife. Only about 10 states actually recognize this law, and they all have different standards. I can understand his issue with his home being on her property, but I would suggest to him that he try to find a different place to put his home if he has no intention on having a relationship with her and her kids.