Please take her to an Urgent Care clinic or the Emergency Room immediately.

Here’s why this is urgent and what it could potentially be (so you can better inform the doctor):

Inability to Move the Foot: This indicates the problem is affecting more than just the skin's surface—it could involve tendons, joints, nerves, or deep tissue. This is not typical for a simple superficial blister.
Severe Pain: Significant pain points to inflammation, infection, or pressure on nerves.
Rapid Onset & Large Size: A lesion that appears overnight and is "huge" needs professional evaluation to rule out serious causes.