"Sie spielt mit dem Herzen."
Indeed, it is. Not only is it weakly declined, but it's also the only neuter noun that is weak - what's more, it's irregular even for a weak noun (the extra -s in the Genitive and the completely unique lack of declension in the accusative singular).
Let me just decline it for you:
- Nominative: das Herz - die Herzen
- Accusative: das Herz - die Herzen
- Dative: dem Herzen - den Herzen
- Genitive: des Herzens - der Herzen
Note: In casual speech, Herz can alternatively remain uninflected in the Dative singular, but not in certain fixed turns of phrase, such as mit halbem Herzen, im Herzen, von Herzen or eine Operation am offenen Herzen (among others).
I really question the use of sentences that verge on nonsensical. In English the only likely meaning would involve another person and be "his" or "her" heart. Surely there's enough trouble learning likely sentences as fending off the confusion of highly unlikely sentences. Is there a better place to post such a critique?