No offence, but the Hindu religion is VERY different from Islam. If the Quran says no, it should stay that way. No matter who it is. No problem so long respect is observed Old coooool pope, no harm. This is also applicable for Muslims.
Nobody can just convert bcos of being impressed by the mosque decorations.
They need to learn more about Islam. There are more important issues to argue than whether its right or wrong for a christian to enter a mosque. I believe as long no offence towards god has been commited, theres nothing wrong with it.