Ask for copies of the social security card (and birth certificate/passport) for each.  Check that the names, DoB, and SSN match perfectly.  The IRS' name control checks the following against the database of SSA -

• First 4 characters of the last name.  Specific conventions apply to foreign names but your tax software should already have taken care of that so long as your input is correct.
• SSN; and
• DoB

If you then determine that everything is correct, your client should call the SSA at 1-800-772-1213 to verify their records.  And if everything turns out to be correct but the IRS continues to reject the return, it will need to be filed on paper.