Cherry helps the Greasers for a few reasons. She doesn't think the fighting is right, and wants the fighting to stop. Secondly, she has a huge crush on Dally. Her feelings for Dally motivate her to do what she can for the Greasers. Additionally, she has met Pony, and through meeting Pony she realizes that not all Greasers are the same - not all Greasers fit the stereotype. There is also an element of drama for Cherry in helping the Greasers - she may very well be taken by the danger and excitement involved in betraying her "own kind".