I am an attoreny licensed to practice law in California. Do not take this as creating an attorney-client relationship. Here is my response to your question and my opinion.

I sympathize with you. I have been detained by store loss prevention personnel when they believed I stole something even though I did not. Quite often shoplifters shop as partners. Because of this, you can find yourslf innocently accused when someone you are with steals something. The reaction of loss prevention personnell in these situations is understandable to a point, but quite often they jump to conclusions and just assume the guilt of both persons in this kind of situation.

Overall, your friend got off very lightly - he could have gone to jail - and nothing really bad happened to you.

You may also get a civil demand letter in the mail stating that you should pay the store money. This is not a criminal matter. In California these demands are often for $500. If you do not pay, the store can take you to court and there is some risk that you would lose and that you would have to pay the demand plus several times that in attorney fees. On the other hand, most lawyers in California tell their clients to ignore the demands. In fact, actual court enforcement actions for these are rare. If you do get a civil demand you will have to make a judgment call as to whether you should pay it, to get it over with, or to stand on your rights as an innocent person and refuse to pay. In my case I paid it, but the circumstances were quite different. Things looked much worse for me and the stakes were much higher. You will have to decide what to do if you get the civil demand, but on the facts you stated I would probably refuse to pay. You might send back a letter stating that you were an innocent bystander who had nothing to do with the theft and that if the store sues you then you will seek attorney fees and will also consult a lawyer about bringing a claim against the store for wrongful initiation of civil proceedings.

I agree with Pandora's Box that you should not shop with your friend again and it would be best to stay away from that store, even when shopping alone.

Good luck to you.