How can you get a copy of a divorce certificate issued in Germany?

My friend's boyfriend was previously married to a German woman. They married in Britain and lived here together for a few years until she left him and returned to Germany. She then applied for a divorce in Germany. She left him with all her debts and he was very depressed after she left him so did not take much interest on the divorce negotiations. He doesn't seem to remember much about what happened during that time.

My friend and her boyfriend are now thinking of getting married but are worried because he cannot find the copy of the divorce certificate that he was sent at the time of the divorce. He says he definitely received it because he had to pay for an official translation of it for his pension, etc. But it doesn't seem to be amongst his documents anymore.

How would they go about ordering a copy of the divorce certificate and who would they contact to do this?

My friend is so worried that they will not be able to get married after all that I am going to try to sort it all out for her. I don't mind if it will cost a bit of money but I am only interested in using official sources - these certificate-finding companies are of no interest to me if there is a way I could get it for her myself.

If anyone has any ideas on what the process would be or even where I could start, I'd be most grateful.