Did you and your friend ever agree to a due date for payment?
With out knowing more, in general, if your "friend" (do you really still call him or her a friend at this point) declares bankruptcy, you will have no legal mechanism to enforce your claim (e.g. you won't get paid).
You can sue, and probably should. I suppose the only issue may be proving the debt. It sounds like this may have been a handshake agreement, but nevertheless; if someone owes you money and doesn't pay, that is what court is for.
That will depend on whether or not he has any assets and his good faith in filing bankruptcy as to whether or not you get paid or can sue in state court to get judgment.
People threaten BK all the time. You will not know whether he is actually willing to do that over such a small debt until you initiate action against him.
I agree with the other attorneys' answers.
You didn't ask this, but it's worth noting that even an oral agreement for a loan, which is what you evidently have, could be proven with writings, if they exist. If you have an email to or from your ex-friend confirming or referring to the debt, or any terms of the debt, that can help you. Maybe there's some other person who knows about this debt because he or she heard your ex-friend talk about it, etc. All that is evidence of the loan that can help you prove the case if and when you file suit against your ex-friend.
A small claim will do. Neither of you will be able to have an attorney with you, but you can get coaching from an attorney in advance if you want, and it really isn't that hard to present your case to the small claims judge.