The article says criminal police had a problem with an extortion email. If the victim is willing to collaborate what's the problem with getting the plaintext?

The article says criminal police had a problem with an extortion email. If the victim is willing to collaborate what's the problem with getting the plaintext? Anyway, I wonder how it's going to be implemented for the case where encryption is done on the client side with open-source tools (not sure if that's the case for Tutanota).