Is a marriage considered to be valid by the Church between two baptized Christians (one being Catholic) if they are married in a civil ceremony? If not, what should they do to correct the situation? Thank you.
In order for the marriage to be valid, the marriage had to conform to the laws of the Church, unless that Catholic party left the Church by a formal act:
Can. 1108 §1. Only those marriages are valid which are contracted before the local ordinary, pastor, or a priest or deacon delegated by either of them, who assist, and before two witnesses according to the rules expressed in the following canons and without prejudice to the exceptions mentioned in cann. ⇒ 144, ⇒ 1112, §1, ⇒ 1116, and ⇒ 1127, §§1-2.
The couple should speak to a priest. Provided there are no previous marriages by either party, it will be quite simple to have the marriage convalidated, or blessed, in the Church.