how can she get a divorce so we can move on with our lives?
He doesn't actually have any choice in whether they get divorced. He can make it difficult for them to get a divorce (for example by constantly delaying hearing by claiming to be ill, appealing every single decision of the court, applying for a change of court and/or judge, etc, etc) and he can string the process out for years, but he cannot make it impossible for them to get a divorce.
can she get a divorce in another country?
In theory she might, but a Polish family court will have jurisdiction over the child custody case. And that is an entirely different matter. While it is in theory possible to have parental rights very limited (or even entirely taken away), in practice it's almost impossible. Just him having been violent in the past won't be enough. If she tries to use the fact that he has had no contact with their son as grounds to having his rights removed, he can just claim that he wants to have contact but she refuses to let him have any contact.
we do not plan to live in Poland, but may live somewhere else in the EU, or the United States
Probably just living in Poland until the kid is 15 is going to be easiest (at that age the kid himself can petition the court for permission to move outside Poland).