Your friend may be a debt collector, as defined under the Cal. Rosenthal Fair Debt Collection Practices Act, which includes creditors of a consumer debt. If your debt was primarily for personal purposes and not towards your business or agricultural, then it is a consumer debt. See the definition at Cal. Civil Code Sec. 1788.2, subdivision (c) for the debt collector definition. In the first link, below, I provided you with the Cal. Department of Consumer Affairs rather lengthy article on what is debt collection and what is illegal. If your friend is a debt collector, then disclosing to other persons the fact that you owe him any money or the status of the loan would be illegal and permit you to sue him for violating the Act. You could then take him to small claims court.
I have in the second link below provided you with my web site which discusses collection harassment and credit reporting issues, including free sample letters that you can send to protect your rights, if this friend comes within the definition of a debt collector.
My main concern is that your friend may not be a debt collector. The friend must be in the business of regularly engaging in debt collection and it specifically excludes attorneys. There is a one year statute of limitations on claims of collection harassment.
Aside from your friends behavior in collecting the debt, if you have a written promissory note documenting this debt, and you're complying with it, then you friend has to adhere to it too, and it the balance isn't owed because you've been late or failed to make any payments, then your friend can't sue you. However, he can sue you if you've breached the terms of your agreement.
But there's a urury issue here. He may only be able to sue for the principal amount, since $400 per month on a $10,000 loan exceeds the 10% per year maximum and is a usurious amount of interest and cannot be enforced, unless your friend meets one of the exemptions, like is a real estate broker and this debt is secured by real property, and this amount probably wouldn't be. Under CA's usury laws, you may be able to sue him to get all of the usurious interest amount forfeited.
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.