its not her responsibility, the other girl. Its his.
She isnt stealing him., he is walking away. Already... think about it.
no one can MAKE anyone do something.
She isnt making him kiss her. He knows he has a girlfriend, he should be respectful enough to not kiss her. But if he likes her more than the other girl, he may just not be mature enough to say so, so instead.. he kisses her. His choice to cause pain. Not hers.
( though I hear your point loud and clear