It is indeed sad to note that there are people, assuming the role of experts in Islamic edicts, issuing directives discouraging their Muslim staff from wishing its Hindu customers "Happy Deepavali" as, according to that person, it was against the Syariat.
To me, such people are narrow-minded and backward-thinking and should not be living in a multi-racial and multi-religious community but in self solitary confinement in caves.
I am not an expert on Islam but I was taught that the Quran instructs Muslims to treat non-Muslims courteously in a spirit of kindness and generosity given they are not hostile towards Muslims. Divine commandments to treat non-Muslims were taken seriously by Muslims.
Allah says: "God does not forbid you from those who do not fight you because of religion and do not expel you from your homes -- from dealing kindly and justly with them. Indeed, God loves those who act justly. God only forbids you from those who fight you because of religion and expel you from your homes and aid in your expulsion -- (forbids) that you make allies of them. And whoever makes allies of them, then it is those who are the wrongdoers." (Quran 60:8-9)
Prophet Muhammad (peace be upon him) himself was the first person to put the divine commands into practice, followed by his caliphs and the general population of believers. The life-story of the Prophet of Islam gives many instances of his kind, tolerant co-existence with non-Muslims. Some of his neighbours were non-Muslims and the Prophet would be generous towards them and exchange gifts. Muhammad (pbuh) would visit them when they fell sick and do business with them.
So what is wrong in wishing the Hindus "Happy Deepavali" or our Christian friends "Merry Christmas"? It is simply a wish, nothing more and nothing less.
To my Hindu friends, Happy Deepavali!