I've been pondering certain things along this line myself. And perhaps
@mrjitty can help answer on this?
I can only talk about my own training - which is not in germany. Things are different here with no adversarial criminal process, but also it's not Mars and similar logic tends to apply in my overvalued experience
My understanding is that if a client directly admits his guilt to his legal representative, then that representative is duty bound to not lie about this. I mean in the sense that, he cannot then defend the client as 'not guilty' against a charge he knows that is true.
In the common law jurisdictions, the lawyer is an officer of the court first and foremost, and thus the lawyer may not mislead the court. Obviously this is a fine line that counsel walks at times.
In the bar training, I was taught that if a client admits guilt to you, you then cannot put an alternate version before the court. If the client demands it, you must therefore remove yourself from the case. This is why counsel is taught not to directly ask the client this question.
But my question is more about what happens if a lawyer discovers information, prior to any charge or trial, that makes him believe that their client is in fact guilty. What (in theory) are the rules around this about them being able to defend that client against the charge in court? Especially if the client is facing more than one charge to be defended, but the lawyer only is 'sure' about one particular charge of which they believe their client is almost certainly guilty of.
In the present case, CB is not even indicted, so I am not sure if FF is officially representing CB in Court in this matter, at this time. If he isn't, then i presume he has much more freedom.
If CB were charged, in your scenario, one suspects FF has to exit the case.
The problem is, it's hard to see how he could represent his client and make the best defence possible, while simultaneously being aware of evidence which proves his guilt.
On the other hand, if he merely thinks CB could well be guilty because the prosecution has a strong case - he can still act.
These can be fine margins
e.g. in the Pistorius case, I believe counsel crossed the line to actually deceive the court about evidence which proved guilt. On the other hand, counsel could have legitimately refrained from engaging on that point, and simply asked prosecution to discharge its burden.
I guess the tldr answer is "its complicated?"
My person opinion is FF wanted the glory of acting on one of the most famous european crimes ever, but likely has zero interest in being involved in the other charges