Cancelling a marriage proposal

Q: This is my friends problem:

I am engaged to a girl. Last week I saw her with her uncle (aunts husband) involved in sex and I also found that she has an affair with 2 other people. My family and the girls family don't know about her illegal relations. Right now I am still engaged to her. Tell me what to do? I can not break the engagement because her family is very powerful. They have money and position. I cannot tell this to anyone because it is difficult for anyone to believe. I don't have proof except that I saw them with my own eyes. Please guide me what to do. Should I marry her or tell me any dua or wazifa which will help me to expose there sin in front of every one. I can't refuse this marriage or run way. 

Previous years qadha fasts and kaffara for breaking a fast intentionally

Q: As Ramadhaan is nearing so I thought of covering up all my Qadha roza but I don't have the exact count of the Qadha roza. What should I do in this case? Also I have Qadha roza of the previous few years and I don't remember how many they are. 

I had once broken my fast when I was young but baaligh, but I did not know that it was a major sin. How do I cover up for that?