Can Muslims wish "Happy Diwali" or "Merry Christmas or "Happy new year" to their non-Muslim friends?
In the Name of Allah, the Most Gracious, the Most Merciful.
No, it is strictly not permissible to say such things. Saying "Happy Diwali" or "Merry Christmas" etc is the practice of the non-Muslims; and the Prophet (sallallaahu alaihi wa sallam) has said, "Whoever imitates a particular nation is from them." (Abu Dawud). The jurists have written that it is extremely detrimental to one's iman (faith) to venerate a day that the non-Muslims venerate.
Also see this answer:
And Allah knows best.
Mufti Faizal Riza
Darul Ifta Australia