Surprised that people argued against what poche said.
A mortal sin requires three elements: grave matter, sufficient knowledge, and full consent of the will. ѕυιcιdє is grave matter, sure, and that's why traditionally ѕυιcιdєs were not buried in church cemeteries. But the other two, we don't know. God may have granted a change of will in view of our later prayers.
And from catechism, there are several ways to cooperate in the sin of another: counsel, command, consent, concealment, provocation, praise, participation, silence, and defense. If someone is driven to ѕυιcιdє by harassment, the harassers could very well participate by provocation, and possibly other ways.