Frankly! I still do not understand why your friend was not able to access the platform using Airtel service. I again checked it and I am again able to get access to the casino. It is kind of weird to me as Airtel is a national service provider and if it has banned the website then it should be applicable for me. I would have again shared the screenshot but you won't again agree with me. As of my understanding, anyone using Airtel mobility connection can get on to the platform. I am not sure whether your friend was using Airtel mobile or its broadband service. I am able to access it with both my local broadband and Airtel mobile connection in my state.
Could it be that there's something to do with some cache that he needs to clear? Or try to restart the modem and get some fresh IP if those ISP that you've mentioned are using dynamic IPs. I'm not from there but there's a possibility that it could be these things when you have no problem accessing with the same provider or it has something to do with the browsers they're using. But if the mirror links are working for them, I guess that's okay then as that's the purpose why they're made for.
Airtel is a mobile service provider in India. They do have broadband service. If they have blocked the website on mobile service then they will block it on their broadband service. I live in a state of India that has access to the big names and I have access to a good local broadband service. Using the local broadband I am able to access bc.games when I use Jio which is a mobile service provider I cannot but when I am using Airtel it allows me.
Maybe they might ban the casino after sometime. India is big country and it has happened in the past when it takes time to ban websites. BC.game has got a good user base in India. If they see that user base in India is shrinking then they would come up with an alternate url.