Hi again mate ,If you have access to his proxies, you can dump the headers and see if it says "squid" or something like that.
I know squid configuration files can become a monster and they're really big and you can route by user in squid (99% sure but i'm not a expert).
I found this > docs.oracle. com/cd/E19575-01/821-0053/6nl4p7bd2/index.html
Do you think that can work ?