As I understand it - silence around sins committed by another would be mortal if as an example someone you knew told you they were planning on holding up a bank ie grave matter, so if you remained silent instead of advising your friend not to do it and offering to help in some way the financial difficulty or whatever prompted their decision to hold up the bank - and assuming they went through with it, then you would be responsible for it too. Because you had the opportunity of preventing someone else from sinning.
It would be venial matter if a friend confided to you they were going to steal a loaf of bread from the bakery (which would have a value of say $3), that would be a venial matter if you did not advise them not to do it, offered to buy them the loaf, thus preventing them from sinninng.
There is also the matter of fraternal correction to be taken into consideration as well. There has to be a reasonable chance that the person will take your advice to heart and change their ways. If it would only make them more stubborn in continuing in these sinful ways, then as far as I understand it, it would be better not to offer correction, as doing so may only harden them in their sinful ways.