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.
It does seem complicated if he really can't access this site and using a mirror link won't help, the bc.game team should be able to explain that he can't access this site, I've never had any problems with the IP I use to access this site even though my country bans gambling sites and many national internet blocks them, so I use the internet from non national can still access it even using a mirror link there is no problem with me gambling on this site in the past few months everything seems easy.
I think there may be other settings that need to be looked at on his friend's cellphone, maybe there is an active VPN which makes him unable to access this site or there are settings from his browser, for example the DNS settings, sometimes when I try to access a trading site using the national internet it can't be accessed and I just change the DNS settings and I can access it again, but I have never tried it on a gambling site, do I need to adjust the DNS settings or is there really another way that can be used to access it, the only way is to carefully check the friend's cell phone, maybe there is something what he uses is either a broadband service or something else that blocks this site. but it's also a good idea to try restarting the modem and getting a new IP