Have she given any consideration to wanting to experience other males so that she could try what she's unwilling to try with you, insofar that if it's a disaster, you'll not be disappointed? She might be reluctant to experiment with you, as you are the one who was patient and took the time to help her past her anxieties. Sometimes it's easier to "let go" when there's not such pressures to perform.
Having stated that, as a hypothesis, the lack of understanding and rules would make any potential experience problematic. While the sage advice of counseling is extremely on point, we hope that you can sit down and work this through. Your concern comes through as a very concerned partner, and it's touching to see that concern. Also, if she's willing to discuss swapping with her friend, that might be a good place to start as to what is driving her motivation to swap. Is it the feeling that she's missed something in her younger years, as suggested, or is there something else that she's wanting for the experience?
Please let us know how the discussion between you is going, and we wish the best as you begin this exploration.