What this person did wasn't only wrong in the eyes of the law, but it brought your church and the name of Christ into disrepute (at least in the eyes of unbelievers). Paul's instructions about church leaders could be applied to every Christian: "He must also have a good reputation with outsiders, so that he will not fall into disgrace and into the devil's trap" (1 Timothy 3:7).
Sad as they are, incidents like this remind us of two truths we should never forget. First, they remind us of our own weakness. When we come to Christ, God forgives our past, and He comes to live within us by His Holy Spirit. But our old sinful nature still lives in us also -- and if we aren't on guard, it will try to take over. We'll discover (as the Apostle Paul did) that "nothing good lives in me, that is, in my sinful nature" (Romans 7:18).
Second, this reminds us that we are engaged in a spiritual battle -- and Satan will do everything he can to defeat us. The Bible says, "Be self-controlled and alert. Your enemy the devil prowls around like a roaring lion looking for someone to devour. Resist him, standing firm in the faith" (1 Peter 5:8-9).
Never assume you are beyond sin's reach -- for you aren't. Instead, stay close to Christ, and ask Him to guard you from temptation. And pray for this person and her family, that God will be their strength in the hard days ahead. Encourage them also as God gives you opportunity.