She is correct. By using any of the 3 laws of cosines (which the Pythagorean Theorem is a special case), it can be shown only one length of the 3rd side is possible. If the 3 sides are designated a, b and c and ∠a, ∠b and ∠c are the 3 angles opposite those sides respectively, by knowing the length of any 2 sides and size of the angle between them, the length of the third side can be computed.