My friend is trying to log in with BYOND, and it won't let him because he's using something called a proxy? I'm not sure what this means, or how to disable it.
BYOND maintains a list of known-bad IPs and prevents abuse from them by completely blocking them. Your friend's using an IP that's on the list (potentially accidentally).

Have them send a request to [email protected], including their IP, so it can be removed from the list.