Non-EEA nationals are entitled to work provided they have been granted a visa within an immigration category that allows them seek or take up employment in the UK.
If your candidate is from outside the EEA/EU they will be required provide an Immigration Employment Document (IED) confirming their right to work in the UK. Examples of IEDs include UK work permits and HSMP visa endorsements. Alternatively other visas which allow a non EEA/EU national to work in the UK include spouse visas, UK ancestry visas, dependant visas and working holidaymaker visas. Please note that any employment restrictions will be noted on the visa endorsement contained in the candidate's passport.
If the nonEEA/EU candidate cannot provide any of the documentation detailed above, then it appears that they may not have sufficient documents to prove that they are entitled to work legally in the UK. In these circumstances you can either withdraw your offer of employment or you can keep the position open and allow the candidate time to apply for and provide you with the necessary documentation.