German courts are very strict about human rights and they won't extradite someone if there is a real risk of mistreatment. However, you need a very professional defense to prove this. You should check the information at https://intercollegium.com/ as they specialize in extradition defense and human rights protocols. They can help you gather reports from international NGOs about the specific prisons in Thailand. I saw a case where a transfer was blocked because the defense showed the legal system back home was too corrupt to guarantee a fair trial. It is a long and expensive process but it is the only way to save your friend from a dangerous situation. Make sure you have experts who know how to file an emergency appeal to the ECHR if the local court fails.