I like a girl and but she is with a wrong a guy who is cheating over her

I like a girl very much and we met 20 days back for the first time and she also liked me and she had committed this in front of one of our common friend but after 15 days she told me that she likes someone else and they are into a relationship from last some days. then we again met for a movie and I came to know that they meet very occasionally as well as they also do not talk that much. I know earlier she was confused about me and that another guy but suddenly that guy proposed her and she accepted because it was obvious because they were known to each other from last 3 months and now suddenly she is talking to me very less I don’t know why may be because she getting confused again which is my perception only. but still I don’t want to loose her after knowing that she like someone else reason behind is that another guy is from a very wealthy family and he is cheating over her. That guy is also involved in a relationship with some other girl too if I will say a single word to that girl about that guy then she will become angry with me and in case if she broke up with him by knowing his reality then she will definitely comes to me but the thing is its very difficult to tell her the reality without any proof. So, I just want a healthy solution to it what to do because still I am in a hope that I will get her because of this I am so tensed and not able to concentrate over my personal and professional life and as soon as I will get a NO I will definitely move on but she is still making me confuse.

i can understand ur situation. i can understand her situation.More or less i have been in place of both of you.See i will advise u to remain completely out of her matters u shud just remain a good frd of hers and as soon as u get a chance when she asks wat u think of that guy u can express ur opinion but not bfor that,becoz if she told him and he asked her to remain away from u then thats a bigger trouble for u,take some time and if she dsnt ask u or u feel like telling her truth then see the ryt time when she is in a good mood means mood to lisn this harsh truth tell her calmly make ur words like "i have no right on ur life but becoz i consider u a good frd of urs i ll tell u that that guy is not sincere with u,dont think that i am saying it out of jealousy,i m just being honest with u becoz i cant c my any frd in pain.Anyways its ur lyf and ur decision.I am just here to help u out whenever u need a hand.Please stay safe,this world is a jungle and u r so innocent''. these words will definitly not hurt HER,anyways dont be obsesed over her we meet ppl and we get attracted but kindly dont take it to heart.She definitely is not much into u as u seem to be.So hold ur horses.Somthing which is in ur fate will be urs otherwise not.However i have a mild guess that u r a yung guy in 20s is it tru?(guessing from ur impulsivness and obsession)