Our 11 year old posts fan fiction on a story site, and strangers comment on every chapter

She has been writing all through the holidays, which pleased me until I found out where it goes. She posts chapters on a fan fiction site under a made-up name, based on a fantasy series she loves. Some chapters have forty or fifty comments, mostly other readers asking for the next part. The writing is good, better than anything she has written for school. That is the difficulty. If I stop it, I take away the one thing she is doing this summer entirely on her own. If I leave it, she is in regular conversation with adults and teenagers I know nothing about. She has said I can read the stories but not the comments, which she calls private. I haven't decided anything yet. I would value hearing from parents who have dealt with this, particularly about the comments.