@Joe Shearer
See, this is what I meant by that post on another thread many weeks back. I don't know if you remember, taking part in religious activities that directly contradict with your own. This is taking it beyond the tolerance limit and I would count it as either appeasement or total annihilation of one's identity and beliefs.
Shirk is literally the biggest Sin in Islam, would it not have been better to just be in attendance with respect instead of taking part in actually performing Aarti? Would a vegetarian partake in a meal of a goat on the occasion his Muslim neighbour or friend invites him for Eid meal? I am certain that he/she would attend, but it would not be expected of them to sacrifice their core religious/social etc ideals just to please another.
EDIT: I am not saying they have ceased to be Muslims, that's between them and God.