Hey again @erdem.law,
Thanks for reaching out to us about your friends' situation.
Just to make sure I understand your correctly - your friends are in Greece at the moment or plan on getting married there? Are they in Greece illegally?
If they do want to get married in Greece, one of the core requirements is to prove you are staying in Greece legally (for example a visa).
Questions around marriage in Greece have been answered before:
Now, if you wish to find out more on marriage in Germany and what documents are required, have a look at this Wefugees Blog article on how to marry.
I think the document you are talking about is the "certificate of no impediment". This is from the Blog:
"...only in the rarest of cases refugees will be able to provide a certificate of no impediment because there home country’s authorities do not issue them.
If your home country does not issue a certificate of no impediment you can ask Standesamt for an exception. They will get in touch with the president of Oberlandesgericht (Higher Regional Court) that will decide on your case. In order to do so you have to provide a non-marriage certificate issued by your home country.
Standesamt might ask for additional documents. That is another reason why it is important to get in contact with them early enough."
If your friends get married in Greece, Germany can recognize it under certain conditions:
Let us know if you have any more questions.
All the best,