This happened to me in v15.7 recently. I found an old post that said you can stop mailservice delete the user's index folder and start mailservice again, that would cause it to redo the indexing for the user.
But this didn't help with mine.. I had this user stuck in indexing for hours and after a while, it would crash SmarterMail. So I turned off indexing service for a while. What seemed to help, was the user finally called
up complaining about issues, so we cleaned up their mail folders, and I was able to turn on indexing after that and all was good again!
So if deleting the index folder doesn't help, it might be a corrupted GRP file or something in there like that, maybe even one of the mailbox.cfg files are corrupted.
I would also upgrade to the latest v16 release, as Chris suggested, as there are some release notes about index fixes.