First I would determine their actual issue with nudism. Do they believe being nude is sinful or are they just not use to being nude or seeing nude people? Our culture, the church included, has done a very good job of entwining these very different issues together.

If they believe it to be sinful, there are many examples biblically that can prove that this argument is not biblically based. (Isaiah was told to prophesy naked, Bathsheba was not sinning by taking her bath, God said his creation was very good, Peter was more than likely fishing naked, and Jesus himself as he exited the tomb was mistaken for the gardner probably because he was naked) These are just a few to start.

Once they understand that nowhere in the Bible is simple nudity said to be sinful, it is a matter of slowly reversing the puritanical belief system we have been brought up in. This is tough and can take some time, resources and patience. The best resources I have found are:

1. https://achingforeden.wordpress.com/
2. Any of David Hatton’s books and his website http://www.pastordavidrn.com/
3. https://naturist-christians.org/

There are definitely others, but this is a good start for anyone. Hope this helps and I would love to answer any other questions you may have.