I didn't see the program since I had to work, however, I doubt she was mentally ill.
I agree with TT, I think it's pure sensationalism to put the woman on TV after she's lost her son, and then she has to deal with her son's actions too.
He will always be her child. She loved him, no matter what he did. Just because your child does something awful doesn't change the fact that he's your child and you've loved, raised, and invested time in him.
I'm certain the poor woman will spend a lot of time asking herself what she did wrong and will have a lot of guilt to deal with. I know I would.
I've read/seen interviews with the parents of the kids who did the shootings at Columbine too, and they blamed themselves. They still loved their kids though.