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.