I don't know your laws, but here in the U.S. there is a law against it called (something like) "contributing to the delinquency of a minor", even if you are the child's parent. You can't even show him the photos you took. And heaven help you if you allow him to show other children.
If this is for real, it should be reported to whatever agency there that is similar to child protective services here in the US.
In the US you don't have to know for sure if child abuse or neglect is happening, just that it's suspected. You can report it anonymously.
(Edit - I thought you said this had been going on for 3 years but reread it and realized you probably meant you'd first had contact with the woman online three yrs. ago; obviously this would be a situation of even more concern if it started when the child was 7.)
If you can't figure out where they live try contacting whatever website you got to know her on. The company that runs the site should probably report anything like this that's brought to their attention, although I don't know if they would.
It might be best to report it as best you can and let authorities there check on the child's welfare and figure out what the situation is. Then it would probably be best to discontinue contact.
I agree that it seems unlikely to have been the child's idea, although he could have a certain amount of curiosity.
I remember college being the first of taking art classes with figure drawing involving nude models; I don't what's current for high school aged students but certainly under age 12 I doubt this would be acceptable.
What concerns me is if she's talking about this on a public forum somewhere, what else might be going on that she isn't talking about? Someone from a social service agency there needs to go to the home and check it out. We just don't know, she could just be misguided in what's appropriate or there could be sexual abuse going on. If there are no concerns then most likely the case would be closed; if something is of concern it needs to be addressed to protect that child.
My background is working with kids and families and what would happen is what we used to call a 'paper trail' (not paper so much now). You document because you don't know where something could lead and if enough adds up that can determine what might need to be done for the child's well being.
I don't think the OP posted a link to a photo; this was apparently discussed on another board or in an online conversation and the OP seemed to be asking for advice.
That thought occurred to me too; I don't know in a nudist camp/resort if there are family areas or if they're adult only. But that would be unusual for someone to live that lifestyle; it didn't seem like that was what was being discussed.
And at 11 years old it's certainly well past the age where little ones may take off their diaper and run around and parents would think it was cute and take some photos etc. This is a mom modeling nude; I don't see what the reason for that would be. If the child was into photography I could see a parent modeling/posing, but why nude? I don't know why someone would do that with an 11 year old boy (much less with her son).
Naturism is far more prevalent and accepted in the UK and on the European continent than it is here in the U.S. Most here are judging this situation by their standard here in the U.S. with little or no real information on what is going on. I am neither defending nor condemning this woman. There is far to little information given to make any real judgment.
I live not far from a mennonite/amish community. While I do not subscribe to their life style and beliefs, I do not condemn them for their long skirts, long sleeve shirts, plain clothing, beards for men hair buns for women, lack of modern conveniences nor their belief that they should only marry within their own community. While we may not understand it, are we in a position to condemn it on such scant information.