Thank you Sir. Sense and sensibility are always welcome.Breaches of that would generally be civil rather than criminal matters, though, and the extent to which EULA terms are enforcable are not settled law. Obtaining a judgement of any use versus someone based in another country would be impractical, especially due to the need to show actual damage to Frontier.
A stronger criminal case (under legislation like the UK's Computer Misuse Acts) would be that the modifications to the client app were causing unauthorised modifications to data held on Frontier's servers. Certainly in a literal sense this would probably be illegal. But in practical terms the steps needed to bring a case (probably including extradition and identification of the suspect) versus the actual damage done would likely mean the authorities wouldn't view it in the public interest to proceed.
Either way in practical terms the courts are not an effective or efficient way to deal with the problem.