ERISA requires that the plan administrator follow the plan documents and pay plan benefits to the beneficiary as determined under the plan. Thus, the determination of who is entitled to benefits under a plan as the beneficiary of a deceased participant depends on the plan's terms. Without knowing the language of the plan, I do not believe anyone can accurately respond to your question.
The responders above all have solid questions and thoughts but without the terms of the plan each is responding with hypos or using assumptions. Note Peter Gulia's response contains "One imagines", "If so," "Many plans," "and no QDRO" with his ultimate conclusion being a question.
You state that "The plan provides that a spouse must consent to an alternate beneficiary designation." If, for example, the plan uses typical wording such as "Any designation by a married Participant of a Beneficiary other than the Participant’s spouse will not be valid unless the Participant’s spouse consents in writing to such designation (which consent acknowledges the effect of such designation, the identity of any non-surviving spouse Beneficiary, including any class of Beneficiaries and contingent Beneficiaries, and is witnessed by a member of the Plan Administrator or a notary public)..., " then a conservative reading of this type of language would be that the 1988 beneficiary designation is invalid because, upon the new marriage (after the filing of the 1988 designation), the Participant's new spouse did not consent in writing to the prior designation (even if a prior spouse had consented to the designation).
Like Peter G... not advice (just thoughts)