Nice to hear from you again!
If your friend is a Blue passport holder, he has been granted an asylum or refugee status. That should also make it easier for him to get married in Germany, if his girlfriend is granted a visa.
First of all, the following Berlin service website explains the process of getting a visa for marriage and what documents they will need to present: https://www.berlin.de/labo/willkommen-in-berlin/einreise/visumsverfahren/artikel.600163.en.php
As the website states:
"Do you wish to get married or enter into a same-sex life partnership in Germany?
If so, the fiancé(e) living in Germany must first apply to a German registry office for a date on which to establish the marriage or life partnership.
The fiancé(e) living abroad will then apply to the German foreign mission (embassy or general consulate) for a national visa for the purpose of marriage..."
Second, have a look at these threads from our platform for more information on the marriage process:
I'd like to learn more about the marriage procedure in Germany as a refugee
How to get married in Germany as an asylum seeker?
On the first thread, Thorgen posted a link to the Wefugees Blog article "How to Marry" in Germany, which can be very helpful for your friend to find out how it works/ what documents they will need.
I hope this helps for now. Please reach out to us again if you or your friend needs any further assistance!