Liz, any Catholic contemplating marriage must be free to marry. To determine freedom to marry, the Church conducts a premarital investigation involving both parties who wish to marry.
Any party who has been previously married will have to give details and documentation during the premarital investigation.
In general, a Catholic who marries an Orthodox marries validly when they marry in the Orthodox Church. The Orthodox have valid sacraments. If the CathOlin didn’t get the required permissions, then the marriage was illicit but it would still be presumed valid.
The Catholic in question needs to talk to their Catholic pastor ASAP.