Are they using their own copy of the database file or is it hosted? (Sometimes even with hosted databases, the "front end" file is on the user's local machine.
If they are using their own copy of a database file, check that file for issues--either replace with a back up or recover it. (replace with backup is "best practice".)
They are using a hosted database not their own copy.
The "front end" file is hosted on a server that they remote to.
Then if other users are not expericing crashes when using the db in identical fashion (This can be a key detail and one nearly impossible to rule out), you'll need to run some tests on that computer. Use a utility to check user account permission settings. Check the hard drive out, etc.
Some of my Windows Xp users have encountered frequent crashes until I gave them a brand new Windows user account to use.
If they are a mac user, try trashing the filemaker preferences file.
Just to be on the safe side, you might take a recent backup copy of the file and run a full recover on it to check for damage. While a damaged file should create problems for all your users, there's always the possibility that the one user having trouble is doing some subtle thing different that is encountering the damage and crashing the file--perhaps by accessing a different group of records than other users.