Is a person sinful if he gives a file conversion program to a friend, knowing that the friend will use it for both permissible and forbidden things, and then the friend uses it for forbidden things?

If you know that the program will be used for unlawful purposes (haram), then you are not permitted to allow it, due to Allah’s saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." It is not permissible to condone the transgression of a transgressor or to assist him; rather, he must be prevented if possible. You can sit with him to guide him toward beneficial content and inform him of the dangers of using the internet and mobile phone for unlawful purposes. Do not give him the program until you are assured that he will not use it for unlawful purposes.