Not content with stealing my boyfriend , she has turned all my friends against me.

= Not being content with stealing my boyfriend , she has turned all my friends against me.

现在分词短语作状语，如果现在分词being为系词时，系词通常省略。

无论是形容词作状语还是beiing现在分词短语作状语，都是表示主语在做谓语动作时所处的状态，在本句中为原因状语。