What can I to ensure that I will get a $50k loan to a friend paid back to me

You should have a signed promissory note for the $50,000. If you can't get the owner to pay it, then your best bet is to file suit to collect it. The statute of limitations in California on an oral contract is two years (which runs from the date the payment was due but not paid), so you're running out of time.

Not only should you have him sign a promissory not but you need to get security in the assets of the business or real estate, if he has any. It is likely that he has competing secuity interests in the business assets, but you should do a lien search (or have a competent lawyer have his/her staff do it) and have the friend execute secuity interest documents and pledge agreements in the equipment, inventory and after aquired inventory, as well as accounts receivable, until the loan is paid off. Additionally, you should get a deed of trust on any real property as additional security if you can. The reason for this is that if the business goes bankrupt, you would get priority as a secured creditor. If you just have him sign a promissory note and nothing else, you are an unsecured creditor. The note needs to be guarunteed by him personally in the event he files personal bankruptcy. It is worth it to have a very competent business lawyer draft these documents. It may put a strain on your relationship but it is better than having him burn you completely by having the debt discharged in bankruptcy. I have seen this happen time and time again in my bankruptcy practice but, when I draft documents, I do what I can to protect my clients in the event a deal goes sour.

You have gotten some good advice about a promissory note and security for repayment of your debt in the 2 answers above.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.