You are asking a very common question. Most of us have wanted to help our loved one get help, because we want a better relationship. We can see so clearly there is something wrong, and we wonder why they can't--or won't. It's part of the illness; people with BPD have very strong defenses in place, such as projection and splitting, to protect them from feelings of inner shame...
I guess I would ask what you hope to achieve. Will telling your mother you think she is seriously mentally ill get you that result?
You might want to check out this topic:
PERSPECTIVES: Telling someone that you think they have BPDWishing you peace,
PF