Section 45. If property, rights or benefits given by will or by law depend upon the election, waiver or other act of a person incompetent by reason of mental illness, mental retardation, or minority to perform the same, his guardian may make such election or waiver or perform such act; provided, that no waiver of the provisions of a will under this section shall be valid until approved by the probate court after notice to such persons, if any, as the court shall deem proper and a hearing thereon, and provided also that if a power is vested in a mentally ill or mentally retarded person for his own benefit, or his consent is required for the exercise of any power where the power of consent is in the nature of a beneficial interest in himself, his guardian may, by order of the probate court, made after notice to such persons, if any, as the court shall deem proper, exercise the power or give the consent in such manner as shall be authorized or directed by the order. If, in any case where the guardian of a ward incompetent by reason of mental illness, mental retardation, or minority has not waived the provisions of the will of the ward’s spouse or brought a petition under this section for approval of a waiver of the provisions thereof and a guardian ad litem has been appointed under section one B of chapter one hundred and ninety-two, such guardian ad litem is of opinion that a waiver of the provisions thereof is for the benefit and for the best interests of the ward, and the guardian fails to take such action after written demand made upon him, such guardian ad litem as next friend may waive the provisions thereof, subject to the approval of the probate court as hereinbefore provided.