I want to lend a friend money for a down payment for a house. How can I secure that loan?

I don't understand your concern about being liable on the mortgage or having your credit hurt.  If all you are doing is loaning your friend money, you are not co-signing her mortgage, there is no reason why you should be liable on her debts or that your credit rating should suffer if she doesn't pay them.  As for the loan you make to her, you can secure it by taking a security interest in her assets, including a mortgage on the house.  The paperworlk to perfect those interests may be something you might want an attorney to handle.  However, you should be aware that it is very unlikely that any mortgage lender will loan your friend money unless it is has the first security interest.  In other words, your friend will not get a mortgage unless you agree that any security interest you have will come second to the bank's.  If your friend defaults on her mortgage, the bank will sell your friend's assets to satisfy the mortgage, and there may not be enough left to cover her debt to you.  There is always some risk in lending anyone money.