What is the ruling on participating with Hindus in their religious rituals and being courteous to them in that, and is it permissible to enter their temples?

It is not permissible to participate with the disbelievers in their festivals, nor to congratulate them on them. What these young people did is a reprehensible act for which they must repent. It may even lead to disbelief if it stems from love and approval of their disbelief. This act is a prelude to falling into their alliance. The claim of mere courtesy is invalid, and Allah the Almighty warned His Prophet, peace and blessings be upon him, against being lenient with them or showing them complacency.