If the user has a gambling addiction is not the casinos fault...
This statement is wrong in some way.
There have been so many cases where casinos/websites got sued becuase they did not offer a proper self-exclusion to their customers.
Yes, you can always "claim" the addict can go to another site and gamble, but the duty of responsibility can attract lawsuits against you, if you fail to honor your regulative requirements (assuming you abide by them and that some regulations apply to you via your license / country of jurdisction etc.)
This means that you decide how much money you will enter into their casino platform.
The amount he deposited/played with is irrelevant - the website can lose its license if the user decides to file a lawsuit against the license holder for being negligible. It seems like many users in this forum believe the websites can do and say whatever floats their boat - but honestly, the OP has a very strong case here if he takes it forward (although for $2,500 it might not worth it but the way they replied to him could attract further damages - it's quite serious).
The website Fairlay doesn't even have a self-exclusion policy, it's not regulated or licensed by anywhere and many people use it - this one cannot really be "sued" because you have no contact details for the site, but Bitsler on the other hand is licensed and a lawsuit against its license holder is possible, but probably not for $2,500.