How do you tell your someone that is in love with his girlfriend, but she really doesn't love him the sameway?

My friend, Eric, is going out with Natasha. Most guys don't realize that she is a really big fake and they fall in love with her when she really doesn't love him the some way. Natasha was talking to our group of friends today about how Eric got mad at her because she was MAJORLY flirtting with another guy. She runs up to him and says I love you, he forgives her, but she talks crap about him when she leaves. Eric is so in love with her, but I don't know how to tell him the truth. HELP PLEASE!!!!