Secret, Silent Screams by Joan Lowery Nixon

It's not suicide. It's certainly not copycat suicide. But Marti is going to need a lot more than her intuition to convince the police of that, and it's not that easy to gather evidence when you're grief-stricken over the death of your best friend, you find out that a lot of people have some very dark secrets, and that whoever may have killed your best friend is out for your blood as well.