A regular polygon all the angles, as well as all the sides, are equal.
Diego's claim is the quadrilateral with four congruent sides is always a regular polygon. He is only mentioning that the sides are equals.
He has not mentioned anything about the angles, the angles may or may not be equals.
In the case, if all the angles are equal too, then the claimed quadrilateral will be a regular polygon as shown in figure(a).
So, for this case, Diego's claim is correct while Mai's claim is also wrong.
While if all the angles are not equal, then the claimed quadrilateral will not be a regular polygon as shown in figure(b).
In this case, Diego's claim is wrong while Mai's claim is correct.
So, overall, considering both the case together, none of them have the correct claim.
So, both are wrong.
