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.
Possible that in some parts that a certain website is blocked by the ISP and in where you're located, it's accessible. But as bc.game was able to solve this problem by providing several mirror links that are really helpful for users that deal with this kind of problem.
It does seem complicated if he really can't access this site and using a mirror link won't help
The guy wasn't able to access through bc.game but he's successfully accessed the mirror links. So with that, there's really no problem anymore even if the main site is inaccessible because there goes the mirror links that are fine and accessible.