Dear @Ny,
Regarding your friend leaving Germany, I am not really able to answer what consequences he has to face. It depends to what other country he would go, what residence status he would have there and whether he would wish to return to Germany at some point.
If he has been in Germany for 5 years, then he could be entitled to the new ‘Chance’ residency permit for people who have a Duldung and have been in Germany at least 5 years.
Have they tried to have their marriage registered in Germany? If their marriage is not recognised by Germany, could they try to instead get married in Germany? Your friend could then possibly in future be able to have a residence permit as a family member if his wife receives a status here.
If he leaves Germany, the only potential consequence I can think of for his wife is if they are living together in an apartment provided for by the Sozialamt. If he leaves, the Sozialamt may then ask her to move out and into a larger housing facility to free up the apartment for other couples/families.
Best,
Éanna