If you read this https://pokemonrevolution.net/forum/topic/5968-url it states "I will preface this thread by underscoring the fact that we cannot case-specifically return any lost items or Pokemon " so they won't help you
You won't get banned because you have his pokemon when he is offline. You will get banned, however, if he comes online, asks for his pokemon, and you refuse to give it back. That's all.