A couple of questions for you.
Is this user using cached mode on a direct connection to the mail server or are they downloading their email to a pst. (personal folder)?
Does the file check happen every single time they open outlook now even if it completed successfully the last time?
If they are directly connected and using cached mode, you can delete the .ost file and let it rebuild. Take the outlook client off cached mode and close their outlook, Do a search (include hidden folders) for the .ost file and delete it. Re-open outlook normally, re-enable cached mode and close and re-open it again to enable the change.
It will take a little time to rebuild the .ost depending on how large the mailbox is.
If they are downloading their mail to a personal folder you can run a repair utility against the .pst file. Do a web search for pst file repair tools.
I recommend these only if it keeps checking the file every time they open outlook regardless if it completed last time sucessfully.
If it opens normally the next time they open outlook after a successful check no action is required. Outlook is working as designed.
A side note – every once in a while my cached mode ost file gets corrupted and I have to rebuild it. I’m used to it and just take it in stride now.