It is considered valid. How do I know? A friend of mine married a man in a Christian ceremony (not Catholic). He ran off some years later. She was Catholic and met another Catholic. She had to go through an annulment for husband number 1 to marry husband number 2. Thus, if the Catholic Church sees a marriage as sacred when performed in a Christian church.. they see the marriage as sacred.
The church is always going to err on the side of caution & require an annulment procedure if there was a prior marriage. But I can just about guarantee that your friend’s prior marriage was found invalid very quickly on “defect of canonical form” grounds, meaning that the invalidity of the marriage was clear as soon as the facts were set forth.