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It would not be against the law to share the photo. The person "might" be able to make out a civil claim for public disclosure of private facts, depending on the circumstances surrounding the photo being sent. If the person sent an unsolicited photo to another person, then there is not going to be much they can do when the person shares it. But if it was part of an intimate relationship and there was an ongoing understanding that the issues were private, then maybe. But even then, the photo would have to be shared with the public at large, like posted on the internet or printed.
Just because someone might be able to sue, does not make it illegal. It is not illegal unless it becomes harassment, like posting flyers all over or at the person's work or something.
But if the person just shares it with a few people, there is probably nothing that can be done about that... this is just the risk carried with trusting someone with a nude photo.