Wow. I wouldn't ever have even imagined that including a bitcoin miner in a program would be illegal. Sure they never declared that they did so but loads of programs do stuff that you'd never know about (for example reporting back personal data about your computer or running complex anti-piracy related code or modifying your system).

I'm a little surprised you can sue for "damaging client's systems and spiking their electricity bills". Don't all video games do that? Hell, even javascript on websites has been known to go into infinite loops and max out my computer for a long time before I noticed. I'd never have thought I'd have a chance if I sued the person who wrote the bad javascript.

Adding something secretly to your employers program without telling anyone however is defiantly something you can be sued over. Wow, sucks to be him or the employer really. I'm a little surprised he even tried, it's not really something that would be easy to keep secret for any length of time.