Part One
Bob can decrypt the text by finding the inverse (assuming it's invertible) of the secret matrix and multiplying it and the encrypted matrix together.
Part Two
It depends which message she wants to know. If it was the one she just saw then there is no need for her to decrypt the message. If it is another one I do not think there is any other way for her to decrypt it.
Part Three
Using Matlab the "secret matrix" is found to be $ \left[ \begin{array}{ccc} -\frac{2}{3} & 0 & \frac{2}{3} \\ 0 & 1 & 0 \\ 4 & 0 & -1 \end{array} \right] $.
So we can find the inverse to decrypt $ \left [ \begin{array}{ccc} 2 & 23 & 3 \end{array} \right] $
The inverse is $ \left[ \begin{array}{ccc} \frac{1}{2} & 0 & 2 \\ 0 & 1 & 0 \\ \frac{1}{3} & 0 & \frac{1}{3} \end{array} \right] $
so the decrypted message is $ \left [ \begin{array}{ccc} 2 & 23 & 5 \end{array} \right] $ which decrypts to B, W, E.
