Just a quick question.
Did the main characters girlfriend reveal the secret to his mother just before her husband visited? So at the end she was just playing along with the charade to keep her son happy?
Also, wouldn’t the East German authorities have intercepted any mail from a defector?
Good film though.
I didn’t get the impression that the mother was ever told (although she’s not stupid and could certainly have figured it out by herself.) Hard to say, since I was in tears by that part. 
The mother definitely figured it out when she got outside and saw the advertisements. It’s possible that she guessed sooner – the explanations given to her were rather a stretch – but played along to keep her son happy.
So it’s a bit ambiguous then.
I thought his girlfriend was trying to tell her what had occured just before her boyfriend arrived with his dad but the scene didn’t last long enough to let the audience know how much she’d revealed.