Alejandra is short-sighted and also cannot focus on very near things…

Step 1. The power. $P = \dfrac{1}{f} = \dfrac{1}{-0.50\ \text{m}} = -2.0\ \text{D}$.

Step 2. The farthest object. An object at infinity gives an image at $v = f = -50\ \text{cm}$. This is exactly her old far point. So she sees objects at infinity clearly, and the new far point is infinity.

Step 3. The nearest object. The image must be at her old near point, $v = -12\ \text{cm}$. We want $u$, so $\dfrac{1}{u} = \dfrac{1}{v} - \dfrac{1}{f} = -\dfrac{1}{12\ \text{cm}} + \dfrac{1}{50\ \text{cm}} = -0.06333\ \text{cm}^{-1}$. So $|u| = 15.8\ \text{cm}$.

What about Tomas? Tomas is wrong. The near point moves farther away, from $12\ \text{cm}$ to $15.8\ \text{cm}$. A concave lens makes a virtual image that is closer to it than the object. The image must stay at $12\ \text{cm}$ or more from the eye, so the object must be farther than $15.8\ \text{cm}$. Her new range of clear vision is $15.8\ \text{cm}$ to infinity.