I don’t know what to tell my sister n law!!!

So my sister n law is 21. She’s dating a guy the same age. They have been dating for 8 months. Her boyfriend has a really good job and blows his money on his car. He went out and bought her a promise ring. Her finger is green bc the ring isn’t real. She took it off bc of her finger changing called him and told him about it. He told her it’s just a ring and to put it back on. Bc he didn’t like her not wearing her ring. I hate this bc she was so excited about getting it. Gosh he could at least got her a sterling silver one at Walmart. They have really pretty ones for cheap. She always ask me what I think about it but I hate to say anything bc I don’t want to hurt her feelings but on the other hand I don’t want her finger falling off either. Another thing he’s really mean to her also.