I have never encountered this so I can’t speak from my own experience but since you haven’t gotten any replies I thought this link might be helpful:
Information about the snail’s shell problems and how to fix them based on existing experiments, and the experience of hobbyists.
aquariumbreeder.com
i did a little reading about this earlier today because one of the nerites I just got in the mail appears to have a substantial crack that has “healed”. From the reading I’ve done it sounds as though snails can recover in many cases, especially if they have enough calcium, magnesium, and other essential minerals available.
the site aquariumbreeder.com where the article link i provided was posted has reputable content in my experience. they appear to create original content from actual applied experience and expertise rather than duplicating generic content for the purpose of generating revenue through product links (like some much of the Internet clutter these days)
good luck - I hope your little friend prevails!