The quadrilateral she plans to construct must be a square or a rhombus, because each side is 5 in. long.
Label each vertex of the quad. A,B,C and D counterclockwise from the bottom left, so side CD parallel to side AB, side BC to side AD. The diagonal AC is a bracing to stabilize the shape of the quad.
Notice the fact that, in a triangle, the sum of the lengths of any 2 sides must be greater than the third side.
But, from the conditions given, AB=BC=5 in. long and AC=10 in. long, so AB+BC=AC. This doesn't agree with the fact mentioned above: AB+BC>AC.
If AC=10 in. long, each vertex A,B,C and D lies on the same straight segment AC, collinear. There certainly are 4 sides, but not 4 inner angles. There never be a quad.
Therefore, her plan won't work well, unless the bracing is cut shorter than it is.