On Oprah a woman sued her husband mistress and won! Is that possible?

A lot in the US will depend on what state they live in and the laws, also even the local areas where judges could be more moral minded.
Often these if go to appeal will not be upheld. Many are based on old criminal and civil codes that are often not held constitutional when they go high enough up in appeals. But then appeals are costly and don't happen often in civil unless it is a large judgeemnt