See this
old thread that discusses a very similar issue. There has been no change to the law in the past decade, so the advice in that thread still stands.
Of course, keep in mind that by your friend having naturalised in the UK, they will have automatically lost Indian citizenship already. They can't renounce British citizenship until they acquire another citizenship. So, in order to renounce British citizenship, your friend will need to first reacquire Indian or some other citizenship and then renounce British citizenship.
And renouncing British citizenship does not revive ILR status. Their ILR was extinguished when they naturalised as a British citizen (as a British citizen can't hold any form of immigration leave). If they wish to reacquire ILR, they would need to start from scratch.
I am very surprised that your friend would not have known that naturalisation in the UK would have meant loss of Indian citizenship. It seems to be very well known among Indian applicants on these forums and I would have imagined that it is generally well known in the wider Indian diaspora.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.