Teach your son a lesson and tell your friend.

Call your friend and explain what happened. If your relationship allows, throw in the sentiment that you really don't want to make her weary of your son, but you feel that you should be honest with her. Then, tell her you would like to teach your son a lesson. See if there is a good time that you and your son could come over for a few minutes so your son can personally explain himself, return or replace what was taken and apologize. If she agrees, set up the time and thank her for her cooperation and understanding. While you're at it, you may want to set up a lunch or something that can just be you and her to balance out the "friend-time" with the "friend-with-a-kid-time."

Then, explain to your son that, in addition to the punishment you are putting him on, he will be personally explaining himself to your friend, returning or replacing (and if replacing, the money comes out of his allowance or money) what was taken and apologizing for taking anything from her home without asking. Take him over there, walk in with him and stand back and say nothing. He must do all the talking and make it right without your help. If he turns to you when you first walk in with any hope that you might jump in and speak for him, keep your mouth closed and nod to him to do it himself. Remember, it's his mistake and his punishment, not yours.

Hopefully, the experience doesn't scare your friend away from kids even more. With any luck, it may help her see that most kids just need a bit of guidance and, in this case, she was a part of helping a child learn a valuable life-lesson.