For a Catholic to receive the Sacrament of Matrimony in an Orthodox Church, I suppose, a “dispensation from form” would be required from the Catholic Church since a Catholic is required to follow the Catholic form of marriage. In addition, I suspose, a “disparity of cult” dispensation would be required from the Catholic Church as well since one of the parties is non-Catholic. In addition, a “decree of nulity” needs to be sought because one of the parties is already married or attempted a marriage in the past.
It’s possible for it to be valid in theory at least, but there are a lot of hurdles to overcome.