First Joni Richardson endured the death of her daughter-in-law, Diff'rent Strokes star Dana Plato, in 1999. Now almost exactly 11 years later, Plato's son, Tyler Lambert, committed suicide at the age of 25.

"It's a shame that such a talented human being would do this with his life," Richardson tells PEOPLE. "He had all the opportunities in the world and we just can't understand it."

According to Richardson, her grandson, a cameraman and amateur songwriter, had long struggled with his mother's premature death at age 36. Plato died of a prescription pill overdose on May 8, 1999. It was around the time of Mother's Day, Richardson notes, making the anniversary that much harder.

Lambert died of a gunshot wound to the head on May 6. At the time, Richardson claims, he had been experimenting with drugs and alcohol, a factor she believes contributed directly to his death.

"He was talking so sweet to me on the phone that morning," she says. "But you never can tell what's in their mind."

She adds that Lambert's body is being cremated, and there will be no memorial.

"Our lives have just been wrapped around him," Richardson says. "It's just devastating."