Welcome to the Wefugees community and thank you for your question.
This is a tricky situation. If we take a look at the legal framework it seems to be clear: Your friend applied for asylum in Greece. Therefore Spain, Germany or any other European country will send him back there because they are not responsible in accordance with the Dublin III convention.
On the other hand, Greece refuses to take back refugees that they are responsible for almost all the time (see this article in German language for example: http://www.fr.de/politik/flucht-zuwanderung/dublin-regeln-der-eu-griechenland-lehnt-ruecknahme-von-fluechtlingen-ab-a-1458320). Coming to Germany and applying for asylum might be successful because Germany will start an asylum procedure on its own if Greece doesn't take him back, but there is no guarantee for that. It's risky and your friend might end up being sent back. However, it's commonly known that the situation for asylum seekers in Greece (and for acknowledged refugees as well) can be really bad compared to other European countries. So I fully understand that you might have the feeling that your friend wasn't treated fairly.