PAXboy - as to the cousin meeting the friend - unlikely if not a breach of the regulations in Germany....
https://www.visitberlin.de/en/import...r-visit-berlin
Extract
Dear Berlin visitors and Berliners,
from Monday, 2 November, stricter corona restrictions will apply throughout Germany. Only if we are all careful together in the coming weeks and adhere to the given rules can new infections be avoided.
- Reduce your contacts to an absolute minimum. Meetings with one other household and a maximum of ten people in total are permitted.
- Please restrict private meetings to a fixed additional household.
- Please avoid private parties.
- Please refrain from leisure activities, private trips and tourist day trips.
- Don't use public transportation if not necessary and avoid staying in closed spaces.
- Only visit people from risk groups if you do not have any symptoms of illness.
- If you have a respiratory disease, call in sick.
- Unfortunately, you can only experience art and culture digitally in November. Of course, we will send you home inspirations from Berlin again this time. Museums, concert halls, operas theatres, tourist attractions, cinemas and clubs will remain closed until 1 December.
- Restaurants, bars, fitness and yoga studios, swimming pools, trade fairs and other event locations will remain closed. Major sporting events will be broadcast virtually and on television.
Similar to England, it would appear that trips "just to visit someone" are banned... I wonder if there are any places to get food in the airport at the moment or if they have all had to close down?
As to passing through "landside" during the layover/wait, I understand that so long as the friend's certificate meets the German requirements (test performed within the past xx hours etc) there would not be a problem. Similarly when checking in for the next flight, reason for travel being that he/she was in transit and already had a ticket for the next flight should be a valid reason to permit onward travel.