Dear Jareena,
Now a days, HR Scinario has drastically changed. Gone are the days to just see and trust anybody's certificate. Furthermore, performance of an individual depends on so many factors including essentially the bringing up of an employee in a trustworthy environment. It is quite a complex phenomena. I have seen so many toms and harrys who were below par performers in x company and meritorious performers in y company after a simple span of couple of year.
Since it is the fact the the employee has worked with your organisation, it goes without saying that he IS ENTITLED to have a certificate to that effect.
Now, it depends on your management, whether they are generous enough to say GOODBY with clear heart or else.
It is a general practice that employers issue and wish bright future to every one UNLESS the employee has been chargsheeted.
rgds,
Arun Dixit