Once your daughters husband converts to Catholicism through the sacrament of Baptism, they both will be free to remarry each other in the Catholic Church as long as there were no previous marriages. If there were previous marriages, the marriages would have to be nullified or annulled by the Church. Convalidation is usually done when there is a Catholic and non-Catholic union. The Church “recognizes and validates” the marriage union of the Catholic and non-Catholic spouses leaving the Catholic spouse in good standing with the Church... free to practice his/her faith.
So what exactly is the big deal about having the Catholic church recognize a marriage?
My marriage is valid and legitimate without the Catholic church condescendingly “approving” it.